Psion said:I think the original question was D&D book not d20 System[/b] book, but everyone is getting confused
So how about it: which single non-core 3e book do you use the most? All I'm considering to be core books are the PHB, DMG and MM -- anything else, from any publisher, is fair game.
Psion said:I think the original question was D&D book not d20 System book, but everyone is getting confused so I'll answer both ways.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.