While I haven't written any books yet, I do write. My favorite application to use when writing is FocusWriter, which runs on the two operating systems I use on a regular basis, Windows and Linux. It also runs on Mac for those who have one and like to use it.
For Windows, download and install the latest version from the website.
For my distro of Linux, Ubuntu, I use the repo version (you need libsdl-mixer1.2 to enable the typewriter sounds):
sudo add-apt-repository ppa:gottcode/gcppa && sudo apt-get update && sudo apt-get install focuswriter libsdl-mixer1.2
I have created a few themes for myself: