It depends on what you mean by "use". If you mean "open and read from", then there are no books that I use every session, not even the PHB. If you mean "utilize a rule from", then there are many.
By the narrowest definition of the latter, I would still say only the PHB is used every session. And that is only because I actually roll (unobtrusively) for any Diplomacy, etc. checks during role-playing.
By a more relaxed interpretation, amounting to what the PC's have immediate access to, I'd say
-Player's Handbook -- duh
-DMG -- once they have even one magic item, the rules need to be at hand.
-Monster Manual -- Summon Nature's Ally
-Unearthed Arcana -- We use spell points and alternate Death and Dying rules (among others)
-Complete Warrior -- Swashbuckler and non-caster Ranger
-Complete Divine -- Spirit Shaman