If by D&D book you mean Wizards of the Coast produced products, I'd say probably Manual of the Planes. If by D&D book you mean any d20 book compatible with D&D, I'd say Monsternomicon although the Book of Fiends gives it a real run for its money.
My fave "books" (i.e. not adventures) would have to be:
the AD&D 1st edition DMG Valley of Dust and Fire for Dark Sun
Third edition Uearthed Arcana The Planeswalker's Handbook for Planescape (makes the 3.5 "Planar Handbook" look downright bad in comparision)
The third edition FRCS