Monergism.com has a superb list of free Christian books available around the internet. It focuses on the books that they highly recommend so you know the books are all well worth reading.
It picks its books from a variety of websites including Christian Classics Ethereal Library, The Puritan Library and Grace Gems.
Amoungst the authors included are John Owen, W. Arthur Pink, John Calvin, John Piper, J.C. Ryle and Jonathan Edwards. So if you are looking for some inspiration for what to read next then you know where to look!