Yeah, campaign settings are definitely the furthest thing from "must buys." They're fairly narrow, defined books, for the most part. I personally love campaign settings - but that's me, and I generally have a good idea of what I want. Campaign settings tend to provide some story and flavor to them, which is why I enjoy them - if I don't use them, I can still probably just enjoy the book for the book itself. But better books that you can use, in most cases, I think.
Outside of the core rule-books, I don't know that there are any "must-buys." More options are always nice, but not really required.
I think monster books are the way to go, though, actually. Books of new magic items aren't really necessary; the DMG has a fairly good system for making new ones all on your own, anyway, and they tend to be less creation intensive as, say, making up a new monster all on your own would be. The same with making up a new spell. Much easier to do than, say, whipping out a 10 CR aberration, between its skills and special abilities and attacks and so on and so on...
Similarly, unless you're getting a class book for every class, you're screwing players of certain classes out of advantages that are opened up to other ones. Even then, in the case of prestige class books, all you're doing is giving an advantage to those who have concepts that fit into what's printed there, while leaving those who's concepts haven't been given a prestige class behind.
If you do go the route of the Complete Divine or Quintessential's or whatever, though, as I said - at the very least, buy one for all the classes. Or at least any you allow in your games (I personally ban Monks - or nearly always do, anyway). I'd consider a grouping like that a potential must-buy, but individually, wouldn't consider any of those books anything of the sort.
Certain option books as Unearthed Arcana or Savage Species might be the place to look; Savage Species doesn't really do much for me, but Unearthed Arcana's something I enjoy in parts. The Expanded Psionics book seems near as close to a fourth core rule-book as they come, but personally, like with monks, I'm not so keen on the psionics. Still, if that's your sort of thing...similarly, there's also the Epic Level Handbook, though that's not always viewed in the best light...
Anyway, maybe not the best of suggestions, but there you have my opinion.