The National Academies—consisting of the National Academy of Sciences, the National Academy of Engineering, the Institute of Medicine, and the National Research Council—recently announced that all PDF versions of books published by its members will be available for free download, starting immediately. This includes more than 4000 existing books, plus any published in the future. These PDFs can be accessed from the National Academies Press web store, from which print versions can also be purchased. They can also be read online for free.
Although the e-books will be available for free, they will still be copyrighted. The National Academies are not government organizations, so their work is not in the public domain.
(Found via Slashdot.)