The 3.5 core rulebooks
DMG, 1st edition
Splatbooks, particularly the Complete series
Epic Level Handbook for the cool monsters
Campaign settings - Greyhawk, Forgotten Realms, Eberron and Dark Sun
The Temple of Elemental Evil
The Glorantha setting for RuneQuest
GURPS Middle Ages 1
Ken Hite's Suppressed Transmission 1 & 2
Cassell's Atlas of World History
Books of mythology and folklore
Encyclopaedia Britannica on CD-ROM
Roget's Thesaurus
Tolkien
Michael Moorcock, especially Elric
Three Hearts & Three Lions, by Poul Anderson
Nemesis, a strip that appeared in the British comic 2000AD
American superhero comics
Morrowind computer game (not a book admittedly but I found it inspiring)