In addition to the e-books availble through Sophi Search, the EBSCO eBook Collection, and ProQuest ebrary there are several websites that offer free access to e-books in the areas of religion and theology. Also be sure to check out Stitt Library's Theological Web Resources page for more sources.
CCEL offers an "extensive collection of classic theological works from all periods of church history.
The Theological Commons is a digital library of over 80,000 resources on theology and religion. It consists mainly of public domain books but also includes periodicals, audio recordings, and other formats.
Search Google's index of books. Some books only have a preview avaialble.
Project Gutenburg offers over 42,000 E-book titles in a variety of formats. Look for the book categories link at the top of the page in order to find the Religion Bookshelf.
"Internet Archive is a non-profit digial library offering free universal access to books, movies & music, as well as 342 billion archived web pages. To search for books, click the Texts link at the top of the homepage.