Aurora's Whole Realms Catalog: This book is just so cool The old-timey looking paper on it helped it look like a fantasy version of a Sears Catalog (the whole point of it), it was much more about "fluff" than "crunch" (the vast majority of items in there had no game mechanic application, and were there just for roleplaying). This was actually the first D&D book I ever bought, and all the little anecdotes and references in there painted such a vivid picture of the Realms that it made it seem like a real world. This book simultaneously made me fall in love with D&D and the Forgotten Realms. To this day, in my 3.5e Realms campaign, this is my real equipment guide for my PCs'.
Runner's Up:
3rd Edition Players Handbook: It really breathed new life into a game that was slowly dying (or at least stagnating and declining). It revived D&D, replacing 20+ years of clunky and confusing mechanics with a vastly improved system that kept the same feel but was much easier to use.
On Hallowed Ground: A great treatise on deities in D&D, it really helped drive home the depth of the multiverse and make the "every belief system is right, even the ones that contradict each other" idea behind Planescape work.
Faiths and Avatars (and it's companions, Powers and Pantheons and Demihuman Deities): Books the likes of which we'll never see again. Dozens of deities across several pantheons written up in great detail, each one with their holy days, dogma and doctrine, history in the Realms, great illustrations of every single priesthood in their ceremonial garb, and almost every one had religion-specific classes, spells or magic items. Like with Aurora's for equipment, this is my real manual of deities for my campaign, regardless of edition.
Expanded Psionics Handbook: I always loved the idea of psionics as a "3rd school" of magic, but every version of Psionics that came before was just bad, imbalanced and insanely overpowered, or actually underpowered and hobbled in 3.0e. The XPH made psionics distinct from magic, but balanced with it, and gave psionic characters unique abilities that you couldn't do with Arcane or Divine Magic (like Psychic Reformation, or implanting new powers through Psychic Chiurgery), just like how Arcane and Divine have the things only they can do and things they cannot do.
Planewalker's Handbook: A work I wish they could make for other campaign settings. It managed to be a largely mechanic-free introduction and summary of the Planescape setting for players, while having enough information for a DM to run the occasional foray into the planes, with great art, and was concise and to the point.
