So I've been playing with Booktype, and I have found 3 reasons to like it over Habitat. It's open source, format agnostic, and (unlike Habitat) you can start using Booktype today.
I'm probably never going to use it much; it's intended for collaboration, not the single user. But I can see that it is well designed for collaboration. There's version tracking, windows for both recent and current activities, shared notes, and status tags for each chapter and section ( new, needs content, completed, to be proofed).
You can add content to a book via copy&paste, typing it in, and uploading files. It has a fairly complete text editor with all the formatting options I would expect. You can also import a chapter from another book on the same Booktype installation.
Speaking of which, Booktype is open-source. You can in fact download the source and set up your Booktype installation on your own server. I have no need to, but I can see how a publisher might want to run Booktype internally.
Output includes ebook (Kindle, Epub, and iPad), Lulu, POD-ready, and text. There re bunch of options for font size, page size, margins, and more.
So I've tried it, and Booktype is not for me. I don't have the need to collaborate, but more importantly I find the editing interface confusing. It looks enough like the standard WordPress menu layout that I'm stumped by the differences. It also behaves different enough that I keep wanting to ask the developers to fix it.
I think I'd be better off with Pressbooks. This tool is built on top of WordPress, and I'm really beginning to appreciate it.