I have to agree that the 3e GURPS sourcebooks were amazing in this regard.  I'd also add Erick Wujcik's Mystic China to the list.

RPGPundit

GURPS Imperial Rome.
that book just gives so much useful info it's almost overwhelming.
this is probably my favorite sourcebook ever.

GURPS Horror 3rd edition.
used the archetypes and story structures for many other systems.

Revised RECON.
i never actually ran or played a game using the RECON system but the info and charts were incredibly useful. hell we even ran Nightlife in Vietnam with this book.


i just ordered Colonial Gothic and i'm hoping to use the info for a Savage Worlds game. although the system looks lite enough i may actually use it.

ditto for Unhallowed Metropolis.

Secret Societies for Nephilim.  A great source of conspiracy ideas for any modern RPG (but esp. CoC!).

KoOS

Quote from: RPGPunditI have to agree that the 3e GURPS sourcebooks were amazing in this regard.  I'd also add Erick Wujcik's Mystic China to the list.

Rats! How did I miss that one?

Yeah, Mystic China got mined by me for 2 different games. And probably more in the future.

Ordo Nobilis (no, not that one! ) for the oft mentioned Ars Magica. Great stuff for feudal games and the nobility - and some nice research relevant to Ars games.

I think i'll add Talislanta 4e, simply because i think it's a super cool book and i'm dying to play it, but haven't had the chance. Not a sourcebook, but still...

More on topic, i guess you can mine it for fantasy stuff.

Shadow Knight (Amber) is the only sourcebook I own that belongs to a game I've never played, so it wins by default although I haven't gotten around to reading it yet. I've also got All Flesh Must Be Eaten and haven't played it, but it's a core book so doesn't count.

Damn, how could I forget the Ars Magica books?  I have a bunch of third edition ArM stuff that has come in useful at various times for ideas, NPCs and campaign writeups.  Also converted the adventure Pact of Pasaquine to AD&D 2nd edition years back and had a barrel of fun...the PCs never solved the mystery, and the druid ended up living in the branches of a tall pine hiding from people in strange masks when he blundered into a fae realm.

I really liked Orpheus' sourcebooks.

Orpheus had the heaviest Meta-Plot of any RPG ever and every book changed the entire setting both in flavor and in scope. I just really liked the huge build-up.

D20 Dark*Matter - mine it for ideas

Rolemaster's Pulp Adventures - I think it could be mined for information for Adventure! or Call of Cthulhu

Can't beat those GURPS Source Books, especially Mayans, Bio-tech, Atlantis, etc.

The Illuminati: New World Order Book by Steve Jackson - never played the card game, but it can give you ideas on supporting cast and conspiracies and adventure ideas (just haven't used it much yet), especially in a Mage or Technocracy type game.

Suppressed Transmission: the FIrst Broadcast by Steve Jackson - lots of weird ideas

Kromosome - the Amazing Engine cyber/bio-punk game.  I've mined for ideas but have never played.

Dungeoneer (Advanced Fighting Fantasy) - I got a lot of use out of it's section on designing adventures.  It's standard stuff by now, but back then it opened my eyes to a few things that I haven't forgotten - I don't use the book at all, but its influence has been massive.

Bard Games' "Atlantean Trilogy" - The Arcanum, The Bestiary, and The Lexicon - were big influences, even though none of my original gaming group ever actually played the game as-is. We mined it for ideas for D&D, which it was mechanically similar to. Today, the Lexicon is a book I still keep handy, because it has a lot of ideas for a campaign world.

"1001 science fiction weapons" was a great sourcebook for d20, even tho I dislike it.

I have the OGL Conan Pocket Handbook (as that what it's called?), and an AD&D 2e book for Lankhmar. I haven't played in either the world of Conan or Lankhmar, but I enjoy fiddling through those books, as they give me ideas for my own campaigns.

The Midnight Circus, oWoD. For some reason this sourcebook just yelled at me of coolness.

It embodied so much of what I did not like about the oWoD, but for some reason I liked it. Will probably never use it though.