Scroll down just a little bit and you should see the orange banner that says:
Find your Kindle Matchbook titles
If you click on that, you should see some titles, if they have already been enrolled in the program.
Both Primary Fault and Shaky Ground have been enrolled in the Matchbook program. If you have already purchased Primary Fault or Shaky Ground in paperback form, you should be able to download the Kindle ebook for Freeeeee! That's right. Free.
I haven't found the feature on Amazon.de or Amazon.co.uk - so perhaps we can expect (as with Autorip) that they are going to roll out this feature at a later date in those stores.
Amazon notified me that my books are enrolled, so if you can't download the free ebooks, please drop me a line.