Books are a great donation and accepted at many resale shops, like Goodwill, Savers, and Salvation Army. Some libraries may take them to resell also, but I recommend calling before bringing them to the library.
Depending on what type of books they are you can donate to senior centers, daycare centers, nursing homes, or hospitals, but again, call first to confirm their interest.
Here are three websites to check out:
More Than Words
One word of caution, no one really wants old text books or encyclopedias. An option is to offer them to theater companies who may like them for props.