1. 2E PHB
2. 1E DMG
3. 2E Monstrous Manual
4. Ultimate Toolbox
5. 1E Manual of the Planes
6. AC9 - Creature Catalog
7. D&D Rules Cyclopedia
8. 4E Rules Compendium
9. 4E PHB1
10. 4E MM2
The $64,000 question for me is whether to go with 4E or 3.5 for the last three books. I like 4E a little better, the content is spread out over a bunch of books, and the Compendium has spoiled me. On the other hand, I figure it's easier to homebrew 4E monsters than 3.X monsters, and monsters are really what I need for the long haul.