Two hardbacks? Because you could get more paperbacks and even more PDFs for the same amount of money...OK, I'll take off the rules-lawyer hat.
Just because I'm super pimping this book, I'd suggest the Book of Iron Might from Malhavoc, for the maneuver and stunt rules. That's if and only if the current combat options aren't enough for you, though, and is probably better used by a DM with some experience.
If you don't want to create your own setting, then a setting book would probably be useful, but the choice of setting is going to be greatly dependent on what you like. Dark fantasy? One of the Scarred Lands books or Midnight. Heroic action? Eberron might be more your speed.
And if you want to do something other than fantasy, get Mutants and Masterminds, which is probably the best superhero system on the market.
But really, it boils down to individual preference. You don't need anything beyond the SRD, assuming you're willing to wing it on character creation and XP awards. In fact, if you're operating off of the SRD, you might find it most useful to buy the PHB and MM to have hardcopy - I know I prefer that to reading the stuff online, and it makes using the stuff during the game a lot easier. Oh, wait - I see you have the corebooks, so you can ignore that.
Anyway, the best (?) thing that you can ask for in a book is that it makes you want to alter your campaign so you can use it - but the books that do that are going to be different for every person. Your best bet is to look through lots of books and see which ones inspire you.
J
