Some publishers (particularly some US university presses) publish freely available open access versions of some of their catalogue of secondary and critical material. You can find lots of eBooks available for free online, but beware - many of these are illegal! Here are some selected legal sites for accessing eBooks:
The Internet Archive is a non-profit library of millions of free books, movies, software, music, websites, and more.
Ohio State University Press are making the complete texts of certain books available from their website. You will need Adobe Reader or some other PDF-enabled program to read the text.
OpenTexts.World is an experimental service that helps you discover free digitised text collections from around the world. Libraries worldwide digitise hundreds of thousands of books. Open Texts brings (some of!) those collections together, allowing you to search across a multitude of different libraries worldwide.
University of California Press
More than 700 open access book from the UC Press eBooks Collection (of 2,000) on a range of topics, including art, science, history, music, religion and fiction.
There are collections of books that are now out of copyright that are made freely available online:
The full text of many classic literary works, including Shakespeare.
This is a digital library of free eBooks. The contents are mostly pre-20th century or early 20th century to avoid copyright issues.
University of Oxford Text Archive
A repository of full-text literary and linguistic resources. Thousands of texts in more than 25 languages.