One aspect of most roleplaying games that I enjoy is the concept of a sourcebook, a supplementary tome usually centered around a single area of expertise or geographic focus. This is a necessity because of the depth of an RPG compared to other types of games. In most computer games or wargames, there is a limited focus on elements outside of the immediate purview of the gameplay, and everything else exists as backstory or context for the main setup. In an RPG, on the other hand, the depth of the concept means that everything can be brought into play at some point, down to the most basic elements of how the universe operates.
To that end, I'd like to analyze some of my personal favorite sourcebooks for various systems across the years, and explain how I think they help flesh out the universe either in a concrete or immersive sense. If you have any books to add please feel free to discuss them in the comments - sourcebooks are naturally improved when there's more of them, after all.
Dungeoneer's Survival Guide (AD&D)
Probably my favorite sourcebook, and to this day a major defining feature of many things I enjoy about RPGs, the "Dungeoneer's Survival Guide" is an essential classic. While it deals in a fairly specific niche (adventuring in underground environments), it brings the kind of design principles to the table that are indicative of a more widespread approach applicable to every aspect of an RPG.
In essence, the DSG deals with every conceivable aspect of underground adventures. This includes the creation and layout of cave systems, proper methods of navigating caves, how to set up mining operations, and so on. It's really in-depth, too, but in a way that relies almost entirely on common-sense logical concepts. For example, the section on mining is really just a list of different types of real-life mining and how one would go about them - from placer mining (i.e. pioneer-style panhandling) to tunnel mining (which requires shoring and infrastructure). I wouldn't say that you could literally learn how to mine and spelunk by reading this book, but the basic concepts are established enough that the details can be comprehended.
In essence, that's the kind of stuff I value when I'm talking about sourcebooks: details. It creates a situation where an unknown is reached and provides an answer for it. If a PC says "okay, well, I want to start up a mine", there's your answer expressed in game terms. If a GM wants to know how a cavern would be set up rather than simply making it "a hole in the side of some rock", there's the answer. It's reference information that exists to round out the nature of the game world and make it so that the GM isn't forced to make up information. That's an incredibly necessary component when it comes to an open-world game, because eventually the choices are going to be limited by the knowledge the DM has access to.
The DSG isn't even limited to that kind of knowledge, either. A full chapter near the back of the book is dedicated to things like story structure, player motivation and rewards, and differing goals for individual players (i.e. the "roleplayer" versus the "power-gamer"). It's detailed enough that I would feel comfortable giving this book, and this book alone, to a potential GM and expect him to be able to craft a decent story from the tips it gives. It's well-written, concise, and full of helpful tips to encourage creativity and expand the player's toolset. By itself, it's probably the single most useful book for a prospective GM to have.
Old World Bestiary (WFRP)
This is a somewhat unusual choice, because as a game book goes, the Old World Bestiary is not that great. It's a pretty good list of monster stats, but other than that it doesn't add a huge amount. It doesn't even really analyze their organization and culture the way that early Monster Manuals did. Really, there's nothing in it that isn't just an expansion of the basic monster types in the core rulebook.
What it does do, on the other hand, is provide in-universe analysis and opinions. This, in fact, makes up most of the book's content. Each creature gets two to three types of data, and all of the data comes from sources within the context of the universe. The "Common View" comes from farmers, traders, sailors, and soldiers. "The Scholar's Eye" comes from specialists and professors, and "Our Own Words" comes from the creatures themselves. This provides a multi-layered in-universe perspective on these different creatures, which to me makes them feel much more tangible. It's one thing to hear an impartial narrator describe a monster's eating habits, but it's another to hear a panicked soldier talking about how that monster slaughtered most of his best friends.
It also provides a few different levels: the "common view" is simplistic and basic but generally provable, while the "scholar's eye" is more in-depth but also more prone to censorship and mistaken assumptions. They also tie into the game's skill system by associating either with "common knowledge" or "academic knowledge", where a successful check on one of those skills regarding a specific monster prompts the GM to relay some tidbit of information from the appropriate category. In my opinion, it's a pretty solid way to establish what that knowledge represents, rather than being a general or abstracted list of facts.
The biggest flaw with the OWB is that it doesn't really convey any new information, per se. There's no revelations about Orcish culture or organization, there's no in-depth analysis of any logistics or supplies. It's all the same stuff, but presented in a new and interesting way. While that's kind of bad for a sourcebook, it's still a good read and helps to present a "worm's-eye view" of the battlefield. It really gives a sense of normal humans going up against titanic monsters, rather than the tabletop game's view of all things being relatively small and insignificant. It tries to convey the fact that in WFRP, you're not some omniscient general, you're a regular Joe or Jane who's about five seconds from being splattered if you annoy that giant any further.
Covering various parts of the Warhammer world, including the Empire, Bretonnia, Kislev, and the Border Princes, these guidebooks create a united effect as each book goes into detail about the specific land it's focusing on. Each book covers the culture, society, government and traditions of its land, as well as the various provinces and states thereof. What I especially like about it is how modular it is: here's a new book, that book covers an area. You can just make a new book for every area. It's simple and reliable and helps immensely when trying to flesh out the setting.
The details tend to be pretty low-key, too, focusing on daily life within the country, as well as local customs, expressions, and superstitions. For example, the Bretonnian supplement emphasizes the difference between the nobility and peasantry, and shows just how separated the two groups are. It talks about differing speech patterns, differing systems of law, and even the underlying mode of thinking that's lead to Bretonnia being a few centuries behind the rest of the world in technological terms. The Empire supplement, on the other hand, tends more towards inter-provincial rivalries like those between the rich westerners and the poorer eastern provinces. It also introduces a lot of potential adventure seeds in the form of different regions and areas with their own quirks and conflicts.
They also go into pretty amazing depth about what people wear, how they act, what they eat, and so on. It's a pretty great source in terms of establishing concepts of everyday life, which is the kind of thing WFRP tends to revel in anyways. Like the Bestiary, it's a really great way to transition from the higher-level concepts of the wargame to the more personal issues of a role-playing game. It's the kind of thing that, honestly, seems pretty necessary for an RPG; how can you set an adventure in a given land if you've only got vague details about it? You need that meat and bone to fill in the gaps, to actually make it feel like a real place instead of just an amalgamation of real cultures thrown into the Warhammer universe. It helps establish the characters there, as well, since any individual is naturally affected by their culture. My one wish is that they'd made more, because there's so much in the Warhammer setting (Tilea, Estalia, Araby, Ind, and the other countries of the fantasy Orient) that would just be rife for adventuring possibilities.
Star Wars, as presented by the movies, isn't a particularly deep setting. Oh, it works for what it needs to do, of course, but as far as details go "the rebels" versus "the empire" might as well be a non-plot. The Empire just naturally has a bunch of Star Destroyers and Stormtroopers and Tie Fighters, and don't worry about where they come from or who's operating them. They're all intimidating and faceless, don't bother thinking about it.
The Imperial Sourcebook, on the other hand, has the somewhat unenviable task of attempting to create a logical, rational government from those vague concepts. It deals with things like the organization, recruitment, and mustering of the Imperial forces, as well as filling in the gaps with regards to other groups and units within the Empire. For example, the sourcebook is the original source of COMPNOR, the Nazi-inspired human-centric movement that formed the basis for many Expanded Universe novels and assumed concepts.
In essence, the Imperial Sourcebook seems like the first time that the Empire was treated like an actual organization or government, rather than being simply a monolithic group of bad guys. There's discussions of how the Empire manages to keep its power and promote its ideologies throughout the universe, and how the common citizen perceives it. It establishes a lot of the more "general use" Imperial ships and vehicles, like frigates and cruisers that fill roles that the giant Star Destroyers can't. It even tries to encompass the absolutely monumental scale of the whole affair, which is, again, not an enviable task.
One odd thing about the Imperial Sourcebook is that a lot of it seemed unnecessary. Not in general, but for the game specifically. Yes, it's good to know that there's a logical backing to the whole affair, but for most sourcebooks the information is relevant in terms of the player. It's important to know how a squad is outfitted because it's perfectly rational to expect the players to run into a squad of Stormtroopers. Once you zoom out too far, though, it becomes more like "are they really ever going to deal with something on that scale?"
This is especially true because it's Star Wars - yes, there's a lot of detailed novels and comics and stuff, but in general when people think "Star Wars campaign" they're thinking about rebels blasting stormtroopers on a spaceship somewhere. They're thinking about iconic, recognizable imagery, not about politics and nuances. Well, at least, at that point it wasn't. The prequels could certainly have used a bit more sense when it came to that sort of thing. I'm not condemning the writers of the ISB, I just think it's kind of odd.
While not actually an RPG sourcebook, the Colonial Marines Technical Manual shares a lot of the properties that I value in a book of that kind. It uses a combination of solid technical details and anecdotal evidence to create an aggregate compendium that is both logical and tangible.
The normal text is standard technical manual stuff - explaining how things work, providing specifications for vehicles and equipment, and so on. However, there's embedded quotes that capture the Vietnam-style feel of Aliens perfectly, giving the sort of quasi-military Cowboy feel that I simultaneously dislike in general but appreciate in Aliens.
The best way to sum this book up would be to say that it's the most natural extension of the Aliens universe I can imagine. It throws out more campaigns, more gear, more battles, and more rough-necked space marines, all while preserving the exact gung-ho theme of the movie. It's jam-packed with the kind of slang, jargon, and general bellyaching that Aliens was loved for. In the same sense, it creates a lot of new scenarios and units that fit well within the wide confines of the series. Aliens takes place in a huge galaxy, and what we see of it in the movies is an infinitesimally small part. Therefore, there's plenty of "room to grow", and the book takes advantage of that in spades.
The one thing that separates this from an RPG sourcebook is, naturally, the lack of rules. In some ways that works to its advantage, because it can give information without having to slow down every five minutes to throw in a bunch of relevant numbers. It's all information that could be used in an RPG system, but it would have to be converted and developed from the data provided. Essentially, though, it fulfills the same function: it expands the universe that Aliens is set in while maintaining the same thematic concepts.
So, To Sum Up:
There's a few things that I think good sourcebooks do:
1) Provide enough detail that a GM can have a definitive answer to most of, if not all of, the situations that would arise within a given game or setting.
2) Introduce new areas and concepts that still fit within the setting's theme and sense of "room to grow".
3) Provide an in-depth look at life from the eyes of the setting's inhabitants, i.e. the people the players are meant to be roleplaying as.
The main goal of a sourcebook, though, should be to answer questions. What those questions are depends on the setting, but inevitably they're going to come up, and it's better for the GM to have an in-depth reference to use instead of having to wing it all himself. Heck, we don't need game systems to do that. "Making stuff up regardless of whether it makes sense" is the primary weapon in the nerd's arsenal.