When it comes to open source office suites Libre Office and Open Office get all the press but they have all the developers. A lesser known (this is the first I'd heard of it) KDE related project called Calligra recently announced a new app for making ebooks.
Calligra has been under development since 2010, when it was spun out of KOffice as resolution of an internal conflict in the development community. The office suite covers all the basics (word, powerpoint, excel) as well as a number of sophisticated apps including ones for editing vector graphics (SVG), taking notes, making flowchart, and of course making ebooks.
Calligra Author is currently not much more than a clone of the Calligra Words app, but long term plans include greater customization of the interface and features. The developers are envisioning their potential users novelists, textbook authors, and others who others who create long and complicated works. And they are planning to support more than simply the writing process; the announcement mentions everything from writing the original synopsis to writing, reviewing, polishing, and publishing it.
I have not tried the app myself, so I'd appreciate feedback. Right now Calligra doesn't offer a stable Windows option, and since I don't run Linux on my computer the office suite is simply out of my reach. Let me know what you think.
I have to say that I'm looking forward to seeing this app be officially released later this year. Right now I cannot recall an MSWord type of ebook creator besides iBooks Author, and that is format and platform limited and thus not all that useful. And while there are a number of plugins for MSWord and Open Office which make ebooks, none offer the direct manipulation of the content from the ebook makers point of view. They just help you turn a Word doc into an ebook, and that's not the same thing.
And then there's Sigil, but that's more of an editor of existing ebooks than it is a way to create them; it also gets far more into the guts of the ebook than most authors might be comfortable with. And before someone accuses me of forgetting Scrivener, I have heard of it and according to Charlie Stross it has several fundamental problems.
In any case, I'm keeping my fingers crossed that the Windows version of Calligra gets done soon and includes Calligra Author.