A second-hand bookstore has popped up in Washington Union Station! Actually, it is a book sale for some kind of non-profit; but the books are real. It is located on the shopping mezzanine, in a rather obscure place. Stop by, hand over whatever you finished reading, and pick up something for the next leg of your journey!
Interesting! It's called Carpe Librem (which if my rusty Latin is to be believed, translates to "Seize the Book!") and is on the second floor of the shopping areas. Books can be donated at the store. They moved the 17th street store down to Union Station.