It made me start thinking about the common types of sourcebooks that have come out for RPGs.
The most common type of sourcebook, aside from the rules expansions, tends to be tied to a location. In fact, if you think about the earliest non-rules RPG books, they were mostly locations: dungeons. And I've been fascinated by them -- now that I think about it, the very first RPG book that I bought (a module) was a location-centered module: T1 - The Village of Hommlet. Locations then expanded to the setting boxed sets and books that dominated TSR & WOTC lines.
Character Option Sourcebooks
Another common sourcebook is one that gives character options. White Wolf really milked this one with the clan books (I think that they're called splatbooks -- dunno why). I think it was an important strategy for them, especially with Vampire: the Masquerade not only trying to break the older stereotypes of vampires (castles, counts, and the Carpathians), but also the more modern stereotype kicked off by the juggernaut series of novels from Anne Rice -- to show what kinds of vamp characters are open for play in a modern setting.
When I think of this type of sourcebook, I think of popular choices like Call of Cthulhu's Masks of Nyarlathotep, Warhammer Fantasy's The Enemy Within, and Shadowrun's epic Universal Brotherhood. But I also think of the Fading Suns shards, the Cyberpunk collection of adventures titled Tales from the Forlorn Hope, and the Over The Edge adventures.
Normally tied into one of the other sourcebooks, sometimes they come out with these: sources of NPCs. Sometimes they're combinations of allies, enemies, and neutrals. Over the Edge had a killer collection of characters -- all with interesting names -- not only in one sourcebook, but also in their CCG!
But sometimes they're all enemies like WOTC's excellent 3rd Edition Forgotten Realms sourcebook Champions of Darkness, or the series of Enemies sourcebooks from Hero Games.
I don't really see that many of these, but it's hard to actually come up with interesting characters in this situation. Super-villains tend to try to cover all bases, but definitely end up with some that will never be used. Is it the same for other genres?
So far, the non-rules expansion sourcebooks essentially break down into
- plot and metaplot
- setting and milieu