As a DM and a campaign author, I find this book pretty indispensable. Whether you play D&D or not you'll find this an interesting read: there are magic items, prestige classes (special, powerful classes that characters can 'multi-class' into as they develop), NPC tables and NPC classes (aristocrat, adept, expert etc), a chapter on generating towns and villages (and their centre of power), a new section on traps (how to design them etc), poisons, clarification of some of the special 'conditions' mentioned in the accompanying players hand book (incorporeal, invisible etc). There's also a new chapter on the planes.
Inexperienced DMs may also like the sections about how to run a campaign, and deal with the players. If you're looking for some adventures, I'm converting my own campaign into downloadable documents.