Front Range Warlock
Oh yeah, the GURPS sourcebooks definitely deserve a mention! Most of them aren't that strictly tied to mechanics, they're just info, so when you want someone else to have done the research for you, you really should give those a look.
Their various licensed IP sourcebooks are some of the best in existence, bar none. For example, their Hellboy Sourcebook and Roleplaying Game, as well as their various Conan sourcebooks are excellent.