Too many spells!
I voted for magic books, really because I think that D&D has too many spells. The PHB has enough to do just about anything that you want, and then you can add on BOEM I, II, III, BOHM, Spells & Spellcraft, Path of Magic, Path of Faith, and all of the Encyclopedia Arcane! (I know I've forgotten some here.)
I'd rather see books on new magic systems. A few of the EA's from Mongoose are cool, but most just churn out lists of spells. Blight Magic looks workable also. Has anyone used this in a game yet?
Carp