One big factor is that RPG books have gotten more expensive over the past few years, and the amount of money I'm willing to spend on RPG stuff has decreased. So in general, I'm having trouble deciding to buy an RPG book unless I think it will be useful as well as a good read. (Or unless it's particularly cheap.)
So, D&D stuff would seem to have the greatest utility, since I'm running a D&D game and all. But finding useful D&D stuff is harder than it looks, because of the circumstances of my game:
My PCs are levels 8-10 currently.
I expect the campaign to be over in another 5-6 levels--probably by the end of 2004.
I've got about enough adventure ideas to finish out the campaign, though I'm having some difficulty fleshing them out.
I've been frustrated enough with D&D that I don't expect to run another campaign of D&D for at least a year or two after this campaign ends.
Fleshed-out adventures that fit my campaign would be ideal purchases. But these are hard to find. By my estimation, less than 10% of published adventures are for characters of these levels (Dungeon magazine runs level-appropriate adventures less than once every three issues), and most of those adventures have their own plots that don't really fit with my plot and my world. (But you know of any published adventures that fit my list, please do let me know.
Another option might be adventure pieces--maps, encounters, monsters, treasure, NPCs. I do get use out of the support of that sort provided on the Wizards of the Coast web site. But there aren't that many books published in this niche, and for most of them, the yield of stuff useful to me is pretty low. Monster books can be an exception; a monster book will usually have four or five monsters that tickle my fancy. I used to quail at the thought of paying $30.00 for a book that would yield me five monsters, but I've become more willing to do so now.
There are, however, a hojillion books published with a bucket of feats and prestige classes. And I don't care. All five of my PCs together are only going to get about a dozen more feats. I don't think any of my PCs are going to take a prestige class, and I have a sufficiency of prestige classes for the few NPCs whose stats are going to matter much. Feh upon all these.
What I really yearn for is GM advice and guidance. If there was a book on DMing that described common player tactics at various levels and how to adjust the game to meet those capabilities, I would gladly pay a lot for that. I know of no such book, though.