It’s a web based tool for formatting and previewing ePubs across devices. Big bonus is the fact that it allows for a ton of things that I couldn’t achieve using Kindle Create. The downside is that it is a little on the pricey side. My thinking is that had I known about this service I would have released my Kindle version months ago. Also the amount of time spent formatting would have been considerably less. Finally, the cost across one book is high. It keeps dropping as I add more books (which I have in the can at the moment).
Based on how it goes with this tool, might also look to use Publisher Rocket as well.