As of today all PDF versions of books published by the National Academies Press will be downloadable to anyone free of charge. This includes a current catalog of more than 4,000 books plus future reports produced by the Press. The mission of the National Academies Press (NAP) -- publisher for the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council -- is to disseminate the institutions' content as widely as possible while maintaining financial sustainability. To that end, NAP began offering free content online in 1994. Before today’s announcement, all PDFs were free to download in developing countries, and 65 percent of them were available for free to any user.
Based on the performance of NAP’s current free PDFs, projections suggest that this change will enhance dissemination of PDF reports from about 700,000 downloads per year to more than 3 million by 2013.
Printed books will continue to be available for purchase through the NAP website and traditional channels. The free PDFs are available exclusively from the NAP’s website, http://www.nap.edu/, and remain subject to copyright laws. PDF versions exist for the vast majority of NAP books. Exceptions include some books that were published before the advent of PDFs; books from the Joseph Henry Press imprint; and in cases where contractually prohibited, such as reference books in the Nutrient Requirements of Domestic Animals series.
[Taken from the NAP announcement]