ChristArt has more than ten Christian books they made available on their website for online reading and free downloading. Most books are for children and can be used in pre-school and elementary Sunday school class.
Christian Classics Ethereal Library
CCEL has a list of Christian writings they made available for everyone to read online. You can do research, study, read, or meditate with the daily devotion. All these you can do on their website.
Worthy Christian Library
Worthy Christian Library has hundreds of Christian books you can read online. Books are arranged alphabetically and according to author.