alibris.com isn't bad either. I ususally go to them for hard-to-find books. Also, most of these bookstores can get you in-print and just-released books for significantly less than you'd pay in a big-box B&N type store.
It pays to shop around though. Prices fluctuate quite a bit. Van Crevald's "Fighting Power" book went from $60 to about $120 in the span of a week.
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