I'm probably a bit late to this thread but I'd like to throw in what we used for developing our publications which allowed us to export to almost any format required;
We use Ubuntu Linux as our OS and all the writing is done using a lesser known program called LyX ( http://www.lyx.org
). For those of you from the 1990's era, you'll get the feeling that LyX is somewhat like WP51 (though not as difficult to get started in
With LyX, we just write and it manages things like hypenation, kerning, margins, widows-orphans, and all those "little" things that collated together turn your hard work into a genuine masterpiece. For novels, we suggest the "memoirs" package.
For eBook generation, we export from LyX to HTML and then import that via Calibre (of which most of you already know and use).
For graphics/artwork we use Inkscape and sometimes GIMP (if we're working on raster images).