How many books are in the public domain?

Answer 1

Public domain books are not protected by copyright. The term of copyright may have expired, they may be works of the US Government and not protected, or the creator may donate them to the public domain.

Answer 2

In the United States, a copyright lasts for 70 years after the author's death, so hundreds of books have fallen into the public domain.

Unfortunately, many publishing companies prefer to retain copyright protection for their publications because of the need for commercial profit. You can use a copyrighted work without permission as long as you rewrite its content in your own words, or at least quote some of it. You do still need to attribute the work, however. Failure to do so is considered plagiarism.