Free kindle book from a choice of six number 1 best sellers (currently this is as per picture) after purchasing a book or ebook. Minimum spend stated as £0.01 when buying a book/ kindle book via Amazon. It seems you will also be eligible if you download and register the free app to an amazon.co.uk account
The promotion code is sent via email within 4 days
Terms and conditions below
"Earn a Free Kindle Book" Terms and Conditions
1. The "Earn a Free Kindle Book" promotion applies to selected Kindle books and runs from 12:01 am on 23 December 2014 to 11:59 pm on 31 March 2015 (the "Promotional Period").
2. If you perform any one of the following actions during the Promotional Period in accordance with these Terms & Conditions: (1) purchase a Kindle book, (2) purchase a (physical) book, or (3) download and register the Free Kindle reading mobile application to your amazon.co.uk account (a “Qualifying Activity”), we will email you a promotional code which you can use to download one Kindle book from a selected range displayed at the point of redemption (a "Qualifying Item").
3. Your Qualifying Activity will earn you one promotional code for the download of one Qualifying Item. Offer is limited to maximum one Qualifying Item per customer and is not redeemable for cash.
4. You have until 11:59 pm on 30 April 2015 to redeem the promo code.
5. Offer may be changed or withdrawn at any time without notice by Amazon.co.uk and has no cash alternative.
6. Offer only applies to Kindle books and physical books purchased at Amazon.co.uk and sold by Amazon.co.uk. Physical books purchased through third-party vendors are not eligible. It does not apply to Kindle books purchased from any other website.
7. The price of the Kindle book or physical book purchased must be equal to or greater than £0.01.
8. Standard terms and conditions apply to use of the Amazon.co.uk website, including the Kindle Store Terms.
9. This promotion is only available to customers who have received a promotional code via email and have their registered address in one of the following territories:
a. UNITED KINGDOM
d. ISLE OF MAN
10. There may be up to four days between the Qualifying Activity and reception of the email with the promotional code.