Many books that have entered the public domain can be found for free online through Project Gutenberg. More recent releases can often be found in public libraries in either physical or electronic editions.
Project Gutenberg is an ongoing effort to digitize works that are no longer under copyright. Over 49,000 free e-books can be downloaded to an e-reading device or read online.
Public libraries can provide access to physical copies of new releases, as well as digital editions that you can wirelessly download through services such as OverDrive. A library card from a participating library allows you to borrow all of these free books.