Nonprofit websites like Project Gutenberg, the Internet Archive, and the Christian Classics Ethereal Library have long offered free books for download, including many classics of theology.  Now for-profit publishers are starting to get into the mix.  HarperCollins is offering complete books on their website, free for the browsing.  (It doesn’t look like you can download them – instead, you browse through the book on HarperCollins’ own website.)  Several works of C. S. Lewis are available, like The Complete C. S. Lewis Signature Classics, which includes Mere Christianity, The Screwtape Letters, The Abolition of Man, The Great Divorce, and more in one volume.

(HT: Freakonomics Blog)