D&D Basic - A one-book game priced at $15.00 that features Fighter, Wizard, and Rogue, with Human, Elf, Dwarf, and maybe two other races or racial variants. A brief spell list, the basics of combat (three or four pages), about fifty monster types, and a handfull of magic items. A good sized chapter on 'how to play this game you just bought' with a discussion of GM styles, player styles, etc. A brief campaign overview, with pull out barony map (a ruined castle battlemat on the other side) and photocopiable charcter sheet, equipment table and spell sheet.
The Complete Feats - All the feats from the past books, even the ones they think now suck. Then a discussion of how to use and not use feats. A discussion of how to gauge feat interaction and illustrations on 'how to break your game into little tiny peices'.
The Complete Prestige - All the prestige classes from past books. Discussions of how to create prestige classes, and how to pick which ones to allow, and which ones to not allow. Meta-discussion on each class and how it plays, and what it's niche is.
The Greyhawk Atlas - All fluff all the time. A history of Greyhawk. Lots of maps commissioned from that guy at Canonfire, city maps, etc. Pure setting material.
The Tome of Greyhawk - All Crunch, all the time. All the deities, PrCs, spells and magic items you could want.
The Campaign Builders Handbook - One of the few 2E books we have no analog for. How to create a campaign world from the ground up, starting with the basics you need for play, then carrying that forward to fleshing it out. Monte Cook writes this, expanding on his DungeonCraft articles and more. There is a seperate map and handout pack available, or downloadable from the web.