In practice, it doesn't seem that bad. It's no worse than having to rewrite parts of your app to submit it to other mobile stores. You just have to make sure you write the actual book outside their tool, in order to be able to sell your book in case Apple rejects it.

In theory, it's an appalling way of doing it. They could have simply put in the eBooks Store rules that you couldn't sell the book anywhere else if it was accepted by them.