Works in the public domain have expired, forfeited, or inapplicable intellectual property rights. This includes books published prior to 1923, works published by the U.S. Federal Government, or other items placed in the public domain by the copyright holder. There are exceptions to this rule, primarily considering orphan works, perpetual copyright and works created by government contractors, but the public domain offers a wealth of history and resources.

Google Books - Google is digitizing books and making all those in the public domain available online to read in full and download. Vast range of subject areas are covered. Limited previews are often available for book still protected by copyright.

At the top of your results list, select Search tools and change Any Books to Free Google eBooks to only get results that you can read in full and download.

Look under the gear in the upper right corner of books in the public domain for download options. This typically includes PDF and/or EPUB.

Internet Archive - The Internet Archive is working to digitize works in the public domain, as well. Fiction to historical texts to academic books.

• Choose Read Online to read the book fully online in Internet Archives' viewer.
• Download options include: PDF, EPUB, and Kindle, among others.

Project Gutenberg - The oldest free eBook site. Works from the classical era into the 1900s. Many primary sources and reference works.

• Read online in plain HTML or choose the Download tab for options including EPUB and Kindle.

HathiTrust - Millions of digital books, journals, government documents, and other volumes, from research libraries. Both public domain and in-copyright works across a full range of subjects are included. All items are discoverable, but viewability depends on each individual items copyright status. 

Perseus Digital Library - large digital library of primary source works in history, science, and the humanities.

• View online in bare bones HTML. Only download option is XML.

Many Universities are conducting their own digitization projects. For example:

• Columbia University Press' Gutenberg-e site allows free access to its collection of award-winning books on history.