Thanks to all for the great replies. I appreciate the help. Although this is a bit off-topic, can anyone recommend a large non-antiquarian used book store in DC or anywhere off I-95 on the southern approach to DC?

I'm a bit spoiled here in North Carolina with the Edward McKay's used book store in Raleigh--10,000 square feet of terrific bargains (mostly non-fiction). I imagine the used book scene in DC might be more akin to the situation in Harvard Square--lots of tiny used book stores clustered together.