1. The Kingdoms of Kalamar Campaign Sourcebook - Obviously need for my KoK campaign(s)...
2. The Kingdoms of Kalamar Player's Guide - Not needed per sé, but an awesome addition nonetheless...
3. The Psionics Handbook - Though it has yet to see use by anyone but myself...
4. The Splatbooks - I don't really care for them, other than for the feats...
5. Manual of the Planes - I've not had to use it yet, but it's good to have, just in case, right?
6. The Stronghold Builder's Guidebook - A very nice addition, if not for the pricing and neat ideas.