OED says e-book.
That's good enough for me
When I am writing anything, I look at possibilities and think hard about the impression I want to give the reader. Ebook, eBook, ebook, and e-book all give different impressions to my mind, and I generally pick the one that gives the impression I want.
Sometimes I will even say "electronic book", just to be damned clear to the audience. You would not believe how many people are unaware that electronic books and print and paper books are two very different things.
That might explain why other folks haven't really settled on a norm, either.