Monster Manual
Monster Compendium: Monsters of Faerûn
Monster Manual 2
Fiend Folio (soon)
Creature Collection 1
Creature Collection 2
Creature Collection 3 (as soon as it's published)
Tome of Horrors
Legions of Hell
Armies of the Abyss
Plus the monster content of:
Manual of the Planes
Deities & Demigods
Tome & Blood (granted, not much)
Book of Vile Darkness (soon)
Oriental Adventures
Forgotten Realms Campaign Setting
Faiths & Pantheons
Races of Faerûn
Magic of Faerûn
Book of Eldritch Might 1-2-3
Book of Hallowed Might
Mindscapes
Seas of Blood
And some other nifties found on the web. In % of thing actively used (met by PCs during an adventure), I'll say we're around 4-5% In % of thing passively used (existing and used to flesh out the world), maybe 10-15%. In % of thing potentially usable (it does exist in the campaign, but it's unlikely we'll use it), I would say around 60-70%.