Project Gutenberg : Offers over 57,000 free eBooks. Choose among free epub books, free kindle books, download them or read them online. You will find the world's great literature here, with focus on older works for which copyright has expired. Thousands of volunteers digitized and diligently proofread the eBooks, for enjoyment and education.

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Obooko: 2500 Free Books

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Open Textbook Library: An open textbook is a textbook licensed under an open copyright license, and made available online to be freely used by students, teachers and members of the public. Many open textbooks are distributed in either print, e-book, or audio formats that may be downloaded or purchased at little or no cost.

Open SUNY Textbooks : This is an open access textbook publishing initiative established by State University of New York libraries and supported by SUNY Innovative Instruction Technology Grants. 

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Open Library: Open Library is an online project intended to create "one web page for every book ever  published". Created by Aaron Swartz, Brewster Kahle, Alexis Rossi, Anand Chitipothu, and Rebecca Malamud, Open Library is a project of the Internet Archive, a nonprofit organization.  

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Digital Book Index :  Digital Book Index provides links to more than 165,000 full-text digital books from more than 1800 commercial and non-commercial publishers, universities, and various private sites. More than 140,000 of these books, texts, and documents are available free.  

WIKIBOOKS Open books: the open-content textbooks collection that anyone can edit. 3,115 books with 86954 pages.