Don't know why I'm on a GURPS memory kick this morning. Back when I used to own a pile of physical books I had quite a bit of GURPS 3rd Edition sourcebooks. Not for playing the game, but for research tools.
Ones I remember off the top of my head are Blood Types, Dragons, Faerie, Monsters, Shapeshifters, Technomancer, Undead, Wizards and a few generic staples like Magic, Grimoire, Magic Items 1 & 2 and Psionics.
I loved to flip through those books for ideas.