The Internet is a gold mine for researchers and avid readers. Open Access Publishing in European Networks (OAPEN) is described as:
… a collaborative initiative to develop and implement a sustainable Open Access publication model for academic books in the Humanities and Social Sciences. The OAPEN Library aims to improve the visibility and usability of high quality academic research by aggregating peer reviewed Open Access publications from across Europe.
For more information, see:
fehrensp. 2011. University Presses. Guide to Open Access Monographs. http://blogs.ubc.ca/fehrensp/2011/07/13/universities/ [accessed: 2012-09-28].
Even more books are freely available at The Open Book Publishers. Some are for digital purchase at cheap prices with hard and paperback copies also available. Alternatively, books for purchase can be read online.
Some university presses are making selected titles accessible such as the three titles listed here:
fehrensp. 2011. Publishers. Guide to Open Access Monographs. http://blogs.ubc.ca/fehrensp/2011/07/11/initiatives/ [accessed: 2012-09-28].
While Open Access does have a high bias for English language publications, two Spanish language books can be downloaded from the Open Monograph Press of the University of Murcia (Spain).
More on the Open Access philosophy can be found on the Go To Hellman blog.