Gutenberg-E (e-books from the American Historical Association and Columbia University Press)
HathiTrust Digital Library - As a digital repository for the nation's great research libraries, the library brings together the massive digitized collections of partner institutions. The initial focus has been on preserving and providing access to digitized book and journal content from the partner library collections.
National Academies Press (More than 4,000 National Academies Press PDFs available to download for free. The National Academies include the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council.)