1) Unearthed Arcana - most read and least-used book in our games, it seems
2) PHB II - I think three out of five PCs in our group use this one.
3) Complete Scoundrel - Spellthief support? I'm there.
4) Complete Adventurer - Love me my spelltheives.
5) Complete Mage - Too cool. Too cool.
6) Monster Manual III - the most commonly used monster book that isn't MM I.
7) Dragon Magic - Just like it.
8) Spell Compendium - It gets used by a few players, from time to time.
9) Complete Warrior - best PrCs in the complete series.
10) Heroes of Horror - just a fun read.