When I was going to get my book converted into Mobi and Ebup formats, I talked to a bunch of people, but Jason and Marina at Polgarus Studio were the best choice. They offered the best prices and the fastest delivery. And even when I wanted a few extra changes, they readily replied and implemented them without any further fees. I would happily recommend their services to anyone.
Making Light In Terezin (Kindle)
Be the Hero of Your Story (Kindle)
Screenwriting From the Soul (Kindle)