Project Gutenberg
gutenberg.org
A volunteer-run digital library offering over 70,000 free eBooks — primarily classic literature and older works whose US copyright has expired — available to download or read online in multiple formats including EPUB, Kindle, and plain text.
Why it is useful
The oldest and largest free ebook library on the internet. If a book was published before 1929, there is a good chance it is here — free, legal, and in a format readable on any device. Covers everything from Shakespeare and Dickens to philosophy, science, and history texts. The catalogue is enormous and the downloads are clean and well-formatted.