Through October 31st, hop on over to Google Play where you may have an offer good for a FREE $5 credit towards the purchase of your first ebook! If you are eligible (make sure you are signed in to your Google account first), you will see a pop-up once you head here.
There is a nice selection of ebooks to choose from. Note that there is a limit of one free ebook per user (offer is limited to the original recipients of the promotional email). To use your credit, just head here and choose an eligible book to add to your cart. You will see the price reflecting the discount at checkout (no code needed).
As just one idea, you can snag The Martian by Andy Weir for $8.99. After your promotional credit, the price drops to just $3.99!