General

A directory of books that can be freely read right on the Internet.
(University of Pennsylvania - US - http://digital.library.upenn.edu/books/) .

The book collection by the NAP is freely readable.
(NAP - US - http://www.nap.edu/)

Directory of Open Access repositories.
(University of Nottingham (UK) and Lund University Libraries (SV) - http://www.opendoar.org).

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Specific

Free access to books about medicine.
(Bernd Sebastian Kamps - US - http://www.freebooks4doctors.com/)

Free access to books about Information Technology (registration required).
(Macmillan - US - http://www.informit.com/)

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