Online Books
Where to look online for the full-text of books on religion, Christianity, and more.
Christian Classics Ethereal
Library
Hosted by Calvin College, this site allows you to search and browse important Christian texts.  Also available are hymn tunes, a study Bible, Bible commentaries, and works of the early church fathers.
Internet Sacred Texts Archives “The largest freely available archive of full-text books about religion, mythology, folklore and the esoteric on the Internet. The site is dedicated to religious tolerance and scholarship, and has the largest readership of any similar site on the web.”
New Advent Contains full text for these books
Catholic Encyclopedia. More than 11,000 articles on Catholic topics.
Summa Theologica. The theological masterpiece of St. Thomas Aquinas.
Church Fathers. Letters, speeches and books from the earliest Christians.
Holy Bible. The Douay-Rheims version, with hyperlinks to the Catholic Encyclopedia.
Project Wittenberg Project Wittenberg is an ad hoc group of individuals dedicated to posting on the internet a cross-section of classic and historic texts written by Lutherans. The project arose from the frustration felt by these librarians, archivists, scholars, pastors and others. Project Wittenberg gives priority to documents written by Luther or about Luther.  Documents by other Lutherans and Lutheran churches or organizations are produced to a lesser extent. Documents are selected and added when a volunteer expresses an interest in working with them.
Sites for Books on All Topics
Barteby.com

“Bartleby.com publishes thousands of free online classics of reference, literature and nonfiction.”  Includes poetry and fiction, as well as the Harvard Classics.  Includes the complete searchable and browsable Strunk’s Elements of Style.

The Free Library

“Since 2003, TheFreeLibrary has offered free, full-text versions of classic literary works from hundreds of celebrated authors, whose biographies, images, and famous quotations can also be found on the site.”
- Suggested websites for further research are also listed.

Google Books

Basic and Advanced searching for scanned images of print books.  Some books are available in their entirety (full view), several disconnected pages (limited preview), or in listings of keywords and phrases (snippet view).  You can limit your search to see only the full view results.
Most full view books are older, out-of-copyright works.  You can search for specific words or phrases when you are viewing a book.

Microsoft Live Search Books

Basic search engine lets you find older, out-of-copyright books.  All books are full text.  Search for words or phrases in the book view.

Project Gutenberg Project Gutenberg is the Internet's oldest producer of FREE electronic books (eBooks or eTexts). Over 20,000 out-of-copyright books on all subjects are currently available.
On-Line Books Page @ U Penn

This browsable listing of online books provides links to the full works hosted by other sites.  Browse by author, title, or subject. 
- This site also lists serials archives.