ChristianAudio offers a free audio book each month — this month’s is Pilgrim’s Progress. You can either browse to http://www.christianaudio.com/free_download.php each month, or sign up for an email reminder.

You click to purchase the downloadable version of the book at the normal price, then enter the monthly coupon code before finishing the purchase. The purchase page shows that you’ll be charged nothing, and it’s been working well for me for many months!

Of course, they also have tons of great books available for purchase!