Google BooksOutstanding source for a survey of current secondary book literature as well as for free downloads of complete works that are out of copyright. Limit your results by selecting "Preview available" under "Any Books" in the left-hand column. "Preview available" books are recent titles still under copyright and commercially available, whose contents can be viewed on a limited basis (generally, four or five contiguous pages). Limit by "Free Google eBooks" for books that are no longer under copyright (generally published before 1925) and can be viewed and downloaded in their entirety. If you cannot find a pre-1925 book available for download in Google Books, search for it at the
Hathi Trust, a parallel database of ebooks with a more generous interpretation of copyright.