It has been suggested to check with your local library. My library takes donations and the "Friends of the Library" sell them in bi-monthly book sales. The profits go back to the library to pay for new books, replacement books and other readable materials. (Not salaries or other expenses) The group makes about $4000-$5000 each sale. It keeps the books in use and the prices are very, very reasonable. Books are 3 for a dollar except for new best sellers that are all under $4.
I donated many of the books from my classroom library and my quilting books in order to thin out my stacks.