Players Handbook --- my original copy (!)
Return to the Keep on the Borderlands signed by John Rateliff, the author, on the title page. I told him of two of my favorite additions, the Bee-Man and the Shy Tower.
D&D Expert Rulebook double-signed by Steve Marsh, near his co-editor credit, and by Erol Otus, who left his famous "EO" tag by his art on the cover.
World of Greyhawk Glossography signed by Darlene on the title page. I picked this because part of her beautiful map is shown on the cover, but she relayed that she had also done the graphic design and layout. She stared at it for a few seconds, remembering her work. I also bought a signed copy of her 1982 game The Battle for the Mid-Realm.
Hope to take pics of these when I get some more time!