Because I'm always curious what tools people use, I thought some readers might be interested in what I'm using to write this book.
Right now I'm using the editor Scrivener, for Mac. This is a great program with many tools that aid in the writing process. It includes things like a corkboard and index cards (for layout out your ideas and rearranging them as you go), an outliner (to help you develop the outline of your document), split screen, and many other ideas that can be invaluable when writing.
I was going to use Lyx with a Docbook template but it got to be a pain in the butt trying to get everything configured correctly. The Docbook template for Mac is only compatible with an older version of Lyx and I couldn't get the version for Linux to import my file correctly.