There are a number of open access and free ebook collections available online. Most provide limited views and some full-text content, usually books in the public domain.
Previews and full-text of books. Full-text books are going to be books that are out of copyright, or books in which Google received permission from the publisher to reproduce.
HathiTrust is a partnership of major research institutions and libraries. The Digital Library is a repository providing access services for public domain and in copyright content from a variety of sources, including Google, the Internet Archive, Microsoft, and in-house partner institution initiatives.
Primarily a large book catalog, it is a project of the Internet Archive. The interface connects you to scanned versions of books from the Internet Archive and connects you to other places to find the books in print.
Project Gutenberg offers over 36,000 free ebooks to download to your PC, Kindle, Android, iOS or other portable device. Choose between ePub, Kindle, HTML and simple text formats.