Most public libraries accept (good condition, general-audience) books from library users, but there are many nonprofit organizations who also accept them.
One of our favorite biblioblogs, BiblioBuffet, maintains an annotated list of such organizations, with hyperlinks to each of their websites. You might want to consult this directory should you decide you want your books to benefit people who have far less access to the printed word than you do.
We’ve put a link to BiblioBuffet’s directory into the “Booklover’s Toolbox,” under the heading Book Donations, so you can quickly find the directory later on.
Atlanta Booklover's Blog 