National Academic Press http://www.nap.edu/order.html
The National Academic Press "publishes over 200 books a year on a wide range of topics in science, engineering, and health, capturing the most authoritative views on important issues in science and health policy." These books are made available to read online for free. Over 2500 books are available.
Digital Book Index : a Union Catalog of Electronic Books, Texts and Documents http://www.digitalbookindex.org/about.htm
Digital Book Index provides links to more than 148,000 full-text digital books from more than 1800 commercial and non-commercial publishers, universities, and various private sites. More than 120,000 of these books, texts, and documents are available free, while many others are available at very modest cost.
Free Library by Farlex http://www.thefreelibrary.com/
Since 2003, The Free Library has offered free, full-text versions of classic literary works from hundreds of celebrated authors, whose biographies, images, and famous quotations can also be found on the site. Recently, The Free Library has been expanded to include a massive collection of periodicals from hundreds of leading publications.
Open Library https://openlibrary.org/
A database with information about millions of books, and a lending library of over 200,000 free eBooks.
Project Gutenberg http://www.gutenberg.org/
"Project Gutenberg offers over 42,000 free ebooks: choose among free epub books, free kindle books, download them or read them online. We carry high quality ebooks: All our ebooks were previously published by bona fide publishers. We digitized and diligently proofread them with the help of thousands of volunteers."
Winnipeg Public Library http://guides.wpl.winnipeg.ca/ebooks
If you have a WPL card, you can access thousands of Fiction and Non-Fiction titles from OverDrive, as well as thousands of always available titles (including some academic presses) on Freading.
Universal Library http://www.archive.org/details/universallibrary
The Universal Library Project, sometimes called the Million Books Project, was pioneered by Jaime Carbonell, Raj Reddy, Michael Shamos, Gloriana St Clair, and Robert Thibadeau of Carnegie Mellon University. The Governments of India, China, and Egypt are helping fund this effort through scanning facilities and personnel. The Internet Archive has contributed 100k books from the Kansas City Public Library along with servers to India.