Google Books includes many full text, copyright free books in addition to copyrighted, non-full text materials. Search the site for a word, author, title, and then select "Free Google eBooks" along the left side to see the full text books.
HathiTrust Digital Library is a partnership of research libraries to provide access to library records and digitized material. Some of the collections are copyright free and made available for free online via the Catalog Search. Select "Full View Only" to find full text material.
Project Gutenberg is a volunteer-driven collaborative project that offers free online books. Books are primarily older works that have fallen out of copyright in the United States. Many titles may be downloaded to eReaders via ePub or Kindle format. The site also includes some audio recordings.