Books: GURPS, GURPS, GURPS

GURPS Deadlands: Varmints for 3rd Edition &
GURPS Space & GURPS Powers for 4th Edition

-Varmints is what you'd expect; lots of weird wacky Wild West critters, from the Bear Lake Monster to the printer's devil. If you're into RPG bestiaries, this one is highly recommended, particularly for being so far off the beaten trail.

-Space and Powers are very typical of 4th edition GURPS; solid chunks of game rules and advice, with a whole lot of math and kinda middlin' art. 4e seems to be locking GURPS into the math-philic demographic. The hard-science world-design system, for example, blithely states that you may need to take fourth roots during the process. Now, they're marketing to a real demographic, and its one that GURPS is strong in. And, it's one that I'm in. But I worry that they're scaring off potential new customers. Supers is also quite math heavy, though it does present an ingenious new way of handling related superpowers. Both are strongly recommended if you like math-heavy rules; otherwise you might want to give 'em a pass.