Every session I use the Dungeon Master's Guide (3.5), Monster Manual (3.5), and the Complete Book of Eldritch Might*.
I use the Player's Handbook (3.5) in about 85% of my sessions.
* Rarely, I'll use the individual Books of Eldritch Might (I, II, and III) instead of the CBoEM, but this is basically the same thing, since very little was done to update the material to 3.5 anyway. We have one player who owns the individual books and two who have the CBoEM (and I have it as DM), so there's never a lack for it around our table.