First, there is a program for Prime members called Kindle First that allows you to choose a FREE book each month! There are six to pick from each month, and even if you don't have a Kindle, the book can be read on the Kindle app on your phone.
Second, do you read book on the Kindle app and use Audible as well? Have you heard of Audio Upgrades? Here's a link that that searches your Kindle library and finds discounted audio upgrades for the books you already own. I found some great audiobooks of favorites for just $1.99!