Official Advanced Dungeons and Dragons, Unearthed Arcana

This is a very valuable resource for anyone that wants to expand the rules for 1st edition AD&D. It contains new classes, including the ever popular barbarian, a few new rules, many new spells, and new non-human pantheons. It also includes new weapons and magic items. Some people don't consider this to be true AD&D because of some variant rules introduced in this book. I've never considered it anything but 'core' 1st edition AD&D and completely necessary.

I've never read it cover-to-cover, but it's seen a lot of action back when I was playing D&D.