Kindle books typically request an Amazon review at the end.
Better marketing would be to ask the reader to open up an email and ramble his thoughts about the book. Right now, while it's fresh! I really want to know what you think and how you felt.
Then, if it's good, polish it up and email it back to the reader with a gift, and request that he take a moment to post it on Amazon.
A small action is more likely to be undertaken than a large one. Reciprocity encourages action. And individual touch for reviews is worthwhile for an author. At minimum, it can be outsourced and automated. Just a few well-written reviews can make a great difference.
Mention how you had your editor polish up the review, same as he does my own writing, which I'd be embarrassed to publish otherwise. If you like it, please consider posting the review, I'd really appreciate it. Feel free to make any changes to it, of course!
Naturally, one should A/B test these calls to action.
Actually, better to induce them to sign up to your newsletter after finishing for a gift, with a code that tells what book they just finished. Then ask for a private ramble review, and do the above.