One of the best places to get free books is also a place to donate books. Many “Friends of the Library” Organizations take used books and sell them to help them raise money for library operations.
Books for Africa collects books that are gently used and acceptable for the African Reader to send to communities, schools, and educational centers in Africa that can use these books.
Along with other donations, the Salvation Army takes donations of books as well. The Salvation Army can be a great place to donate your books when you do full de-cluttering of your house.
Goodwill takes clothes, household items, furniture, and used books! They sell them in their stores to raise money for their charity. They also provide a tax deduction receipt.
Ask your local shelter or food bank if they take donations of used books. Specifically used children's books may be well received by those utilizing these services.