Ah I forgot about the Canda thing *fail*
There's also Bookmooch.com - same principal but international.

I know what you mean about giving up books though! I am absolutely a book hoarder! and there are some 'keepers' that I'll never give up But I read so much and collect so many that there's no place to put them all! I got over keeping books I don't love a long time ago - they take away shelf space for books I do love!