Found this a few days ago and found it to be useful for finding free e-books or heavily discounted ones. Bit of info from the site below
How it works
BookBub alerts you to limited-time free & discounted ebooks matching your interests.
It's free to join, you can read on any device, and the books are yours to own.
Shows books from Amazon, Kobo, iTunes, Nook and Google Play.
BookBub is a free daily email that notifies you about deep discounts on acclaimed ebooks. You choose the types you'd like to get notified about — with categories ranging from mysteries to cookbooks — and we send great deals in those genres to your inbox.
BookBub doesn't actually sell books. We simply alert you by email to fantastic limited-time offers that become available on retailers like Amazon's Kindle store, Barnes & Noble's Nook store, Apple's iBookstore, and others. Book publishers offer deals at these sites for promotional purposes, and our staff works with them to determine the best ones to feature to our members.
BookBub features ebooks ranging from top-tier publishers to critically acclaimed independent authors. Our team of experts makes sure that we're only featuring great deals on quality books that you'll love.