Ok, here would be my top 5 list of books you should look into.
1. Conan. No doubt about it, this book is the shiznat. No other book has been released since the d20 movement began that has captured my imagination like this one. Lots of rules variants that are extremely cool, great cultures, cool low-magic classes, and an inspired dynamic world. Most refreshing of all, no elves, orcs, underdark, or hordes of monsters running around! This book fixed most of the rules problems I've had with 3E/3.5 as well. If you like Howard's Conan stories at all, you can't pass it up. Some might gripe about its $50 price tag, but its well worth it- Hell, its probably worth $75! Its also going to be EXTREMELY well supported. I don't typically like published settings and won't run them, but I WILL run a Conan game in the near future. Its that good.
2. Book of the Righteous. Some might say its too specific since its got a fully detailed pantheon, but I have used tons of aspects from this book in my homebrew world of 12 years. LOTS of inspirational material in this book, and the best pantheon of gods out there anywhere. It also has the Holy Warrior class, a paladin more customized to his patron deity in his abilities.
3. Book of Fiends. Technically its not out yet (should be by Friday), but I looked at an advance copy at my LGS. I loved Legions of Hell and Armies of the Abyss- and this book expands on those. Its designed to be used with Book of the Righteous, but works really well on its own too. There are lots of great fiends that are not very typical D&D-esque (which to me is a good thing). Besides, who can't use more fiends?
4. Midnight. Probably the best executed and most concise campaign setting out there right now. This is a truly inspired book, and one that can give you tons of ideas for your own game, even if you don't run it specifically. This is also the only other published setting I have run (although it was only a 10 session campaign

).
5. Unearthed Arcana. This is the new book by WotC, NOT Arcana Unearthed. I just got UA last Friday, and have read through about half of it (been too busy with Conan to devote more time to it), but this book has something for every campaign. Basically its a ton of rules variants, many of which add a lot to the game and are superior to the way 3E/3.5 handles things. Worth a look, although probably not terribly inspirational for generating new campaigns or ideas.