I guess I would have to come back with "define core."
If you mean the books I accept as integral to the game, right now its Pathfinder RPG, MM 1 - 3, DMG 1 & 2, PHB 2, Fiend Folio, Expanded Psionics Handbook, Tome of Horrors Revised edition, Tome of Horrors II - III, Book of Fiends, and...