Baen Free Library
Focused on science fiction and fantasy, this site provides freely-available e-books for us in multiple formats, including the ability to read online.
Many libraries provide e-books from a company called EBSCO. These e-books are non-fiction and can include literary criticism, career information, and social issues. The selection of e-books will be different from library to library.
Google Books includes e-books on almost any topic. Depending on the copyright status of the e-book, you may be able to download it or see just snippets of it. You also may see links to purchase the e-book or find a physical book in a library.
The e-books in HathiTrust include parts of the Open Library and Google Books collections. In addition, the partners in HathiTrust include their own digital collections. Depending on the copyright status of the e-book, you may not be able to download it.
The goal of this site is to provide a web page for each book ever written. Most e-books are available in many formats. For those that are not, links are available to purchase the book or borrow it from a library.
Project Gutenberg includes primarily public domain e-books which are available to download in multiple formats.