Free PDFs of more than 4000 titles are available from National Academies Press. Subject coverage is very broad. This is what they say on their website:
“The National Academies—National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council—are committed to distributing their reports to as wide an audience as possible. Since 1994 we have offered “Read for Free” options for almost all our titles. In addition, we have been offering free downloads of most of our titles to everyone and of all titles to readers in the developing world. We are now going one step further. Effective June 2nd, PDFs of reports that are currently for sale on the National Academies Press (NAP) Website and PDFs associated with future reports* will be offered free of charge to all Web visitors.”
You can browse the titles available: http://www.nap.edu/
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