Dungeoncraft

From the back of the book:

The definitive d20 System resource for dungeon design and survival.

• New rules for characters adventuring in dungeons including prestige classes, feats, equipment, and dungeon tactics and survival tips.

• Detailed guidelines on the role of magic in dungeon adventures, along with new spells and magic items.

• Comprehensive rules for environmental hazards and encounters.

• A complete, step-by-step dungeon design system.

• New rules for monster morale and the social geography of dungeons

• Several unique encounter areas, such as the Chamber of Winds and the Machine.