I start out in google docs because it's convenient and the change history is nice to have. I do this instead of plain text because I like adding some formatting as I write. If the book gets over 200 pages (or thereabouts) google docs starts to have problems and then I switch over to Pages on OSX. There, I save edits to google drive as I go. When it comes to (self) publishing time, I actually end up typesetting in word (for print) because it's the easiest, least horrible tool for that (though still horrible). HTML for kindle.
