Deities & Demigods 14 Seriously, did any of you upvoting this actually get much usage out of this book compared to the others here?
Fiend Folio 8 (...cos we used this a lot, including at least twice tonight....)
Monster Manual II 4 (....as I've just got home from a 1E session DMing UK5 Eye of the Serpent, and it used several creatures from this - Crows/Ravens cost our Druid an eye!)
Unearthed Arcana 10 (....and the Monk playing UK5 decided to be proficient in Lasso, and the fighter opted for weapon specialisation from this book)