It’s not often something is offered for free, but many many books that are in the public domain are available for free online. The complete works of Shakespeare. The book Twelve Years a Slave. Pride and Prejudice. Candide. All the Sherlock Holmes stories. The key here is public domain.
What is public domain? According to the U.S. Copyright Office, public domain “is not a place. A work of authorship is in the ‘public domain’ if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.”
When does something fall out of copyright and become part of the pubic domain? A very general rule of thumb is the life of the author plus 70 years, however, the actual term can be quite complex depending on when something was written and what it is (book, work of art, sound recording etc.) This great resource from Cornell University explains the life of copyright in a handy chart: Copyright Term and the Public Domain in the United States.
And though many of these works are available online, it’s important to realize that MOST content online IS protected by copyright by default. Unless specifically noted one should always assume a work is still under copyright, whether in print or online.
So where can I get these free eBooks? Two great sources are gutenberg.org, and The Public Domain Review. Google Books also has full text of many books in the pubic domain but beware – most books you find in Google Books are still under copyright (which is why you can only see parts of the book).
If a text available in the pubic domain is required reading for a class, you should ask your professor if reading the eBook is OK – they may want you to read a specific edition of a book for things like the introduction, annotations, or footnotes.