Gale Virtual Reference Library
Selected reference books, including subject-specific encyclopedias, biographies, social studies and histories.

Internet Archive : Text Archive
The Open-Access Text Archive is a searchable collection of over 30,000 titles..

NetLibrary
Over 3,000 titles covering subject areas in education, psychology, science and social sciences as well as business, computer science.  Some reference books as well..

National Academy Press
A continually growing collection of over 2000 titles on a wide range of topics in science, engineering, and health

Google Book Search
Searches the full text of 10,000 public domain books that Google scans and stores in its digital database.