Several locations in the District take used books for resale. Or check with your local library and your local used bookstore in the Yellow Pages under "Books-Used." Some transfer stations do not accept books although Springfield, Weathersfield, and Ludlow do in their swap shops.
If they are not in reusable condition, check with your transfer station to see if they're recyclable.
Another option - Books To Prisoners is a Seattle-based, all-volunteer, non-profit group that sends free books to prisoners in the U.S.