Ebook Central contains over 125,000 academic book titles covering a multitude of topics.
View the Ebook Central QuickStart Guide for more information
Pierce's Ebsco eBook collection contains over 75,000 academic books on a variety of topics.
View the Ebsco eBook User Guide
Gale Virtual Reference Library is composed of online Reference books. GVRL is a great place to start to get an overview of your topic.