Haven't heard of Hathi? Give it a try! HathiTrust Digital Library is a digital preservation repository and highly functional access platform. It provides long-term preservation and 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. Bibliographic and full text search are available for all volumes in HathiTrust. Public domain volumes are freely accessible to the public and can be downloaded in their entirety with authentication by persons affiliated with partner institutions. NYU Libraries is a HathiTrust partner institution.
If you use an ebook reader such as Kindle or Bluefire (or if you just want to read on the web), there are many sources of free electronic books. Here are some recommendations:
And if you like audiobooks, check out Librivox, a project to create free audio versions of books in the public domain.