Forgotten Realms Campaign Setting (Dungeons & Dragons d20 3.0 Fantasy Roleplaying, Forgotten Realms Setting)
This book contains nearly everything you need to run a bare-bones version of Forgotten Realms: it details various regions, cities, people, world, cosmology, monsters, deities and other items of interest, such as NPCs people have learned to love and hate.
It also has new rules, prestige classes and feats that are Faerûn-specific, but which can be ported to other game worlds as well.