Note: IF the "Kindle price" is still $0.00, use the "Buy Now" button -- that way, you'll actually own the book and you can read it anytime in the future. You DON'T have to subscribe to Unlimited.
It's the Kindle ebook version, but you don't need a Kindle to read it. Amazon has a free Kindle app you can download to read it, or you can read it on your computer using their free Kindle Cloud Reader in your web browser.
I hope you can grab the ebook while it's free. I think it's regularly $37.
However, if you miss out on getting your own free copy, you can still borrow the ebook for free anytime if 1) you are an Amazon Prime member and you own an Amazon Kindle or Fire device, or 2) you subscribe to Kindle Unlimited (you don't need to own a Kindle or Fire device for this option).