As of June 2011, all PDF versions of books published by the National Academies Press will be available via PDF at no charge. This includes the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council.
Please read the press release from the National Academies Press.
Some of the topics that are covered include, Behavioral and Social Sciences, Health and Medicine, Education, Biology and Life Sciences, Computers and Information Technology — and more!
Here is a sample of books that are now available in PDF format: