Google Books
http://books.google.com/
Search and preview millions of books from libraries and publishers worldwide. It includes books with expired copyrights or with the author’s permission to be in public access. Search of other books is limited (table of contents only, particular chapters or single pages).
Yahoo Books
www.yahoo.com
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Search of books from all subject areas with expired copyrights or with the author’s permission to be in public access. For other books it is possible to receive basic bibliographic data or table of contents.
Project Gutenberg
www.gutenberg.org/catalog/
The oldest website with books with expired copyrights.
Classic Reader
www.classicreader.com
Great collection of classic texts.
Digital Library
http://digital.library.upenn.edu/books/
Million of books from all areas.