I love this service as a publishing solution. You can write markdown and get your book updated to dropbox (while you write it/push it to VCS) and then publish directly through them to epub/PDF. But the other nice thing is they can export Amazon-compatible files so you can publish to Kindle.
They also offer a very fair and transparent royalty structure. I found Amazon could be a bit greedy. The reason why is when a book goes above 9.99, they increase the royalty significantly.
The only other tidbit I have to say is:
I also considered using sphinx-doc as a route to build the book. I haven't found a way to offer an appealing LaTeX (theme? template?) that'd work with it. If you do know LaTeX well, and have time on your side, maybe sphinx-doc could be down your alley. It can build to HTML, PDF, and epub.
As far as distribution goes: My experience with Apple's book store was very cumbersome. Difficult to get support, and in one case there was a glitch that unpublished the book.