The local public library accepts donations of books in decent condition. Some become part of the library's collection for circulation. Others are used for book sales to raise money for future acquisitions. A receipt is available for the asking, and such donations are tax deductible. Check tax guidelines for how much can be deducted.
Local schools may have libraries, and some of the books you have available for donating may be appropriate to those libraries, particularly books of Jewish interest for yeshivot. Ask the school.Three local high schools will take donations of books for their libraries but ask that you query first as to type of books/title of books that they can use.
Senior citizen centers and nursing homes also accept donations of books in decent condition for the use of their clients. The hospitals in our area have books that they offer to patients to read. They, too, accept donations of books in decent condition.
Many community centers, both Jewish and secular, have libraries and appreciate donations of books in decent condition, as well as books that are fairly current.
A local women's and children's shelter asks for book donations, particularly children's books.
If you have textbooks or academic books that are of fairly recent vintage, some colleges will take such books as donations to the library. Ask first.
Craft and how-to books are a good fit with senior citizen activity programs.
Ask a friend if they have read author X and if they are interested in some freebie books.
There are organizations that take used textbooks and send them overseas to countries where money is in short supply and used textbooks are welcomed with open arms. Ditto with some regular books. Check them out on the Internet.
If you teach and you find yourself with more resource books then you can keep in the house, ask your school if they have a space where teacher resource books can be kept, and donate them. Failing that, ask some of your fellow teachers, particularly the younger ones, if they would like some resource books.
If it turns out that the books aren't usable any more or no one wants them as a donation, don't just toss them into the trash. Paper is recyclable.
It hurts me to have to give away a book, but the pain is lessened when I know the book is going to a good home that will love it and use it. Think of it as doing good while cleaning up.