Monster books.
1e: Monster Manual, MM2, Fiend Folio, all modules from 1e (new monsters that havent seen print in any book- like the ones in the original Palace of the Silver Princess or a few from S4 that never saw print).
2e: All Monstrous Compendium inserts, both binders, the hardback book (Monstrous Manual- both white and black covers)
3e: Monster Manual, Creature Collection I, Creature Collection II, Denizens of Darkness, Oriental Adventures, Manual of the Planes, Epic Level Handbook, Creatures of Rokugan, Monsters of Faerun, Minions: Fearsome Foes, Armies of the Abyss, Legions of Hell, Tome of Horrors (I wrote it, so I have the manuscript), and about 200 (so far) new monsters that I have designed since I did the manuscript for Tome of Horrors.
Also- I have at least one hard copy of every single issue of Dragon (not the CD ROM collection) ever published including the Strategic Review, so I have access to all the monsters in the new and old mags that havent seen print anywhere else.