my oldest book would have to be my twilight 2000 core rulebook from the 80's
I also have the warhammer 40k rulebook, wepons book, and a newer (the black one with revised rules) rulebook and an imperial guard codex.
I got sick of using other people's books so I finally got the 3.5 ed. player handbook, DM guide, and monster manual. I also have the epic level handbook, the complete rogue suppliment, and the FR Underdark suppliment.
I also have many rp e-books, folders and folders of them, and a few CD's worth of DnD quests. And I have a couple notebooks of DnD campaign notes and a couple of character journals. I also have a full notebook (five subject) of my NWN character's journals, encounters, and important events (not to mention novel-like backstories