What software tools are out there to help me write a book?

By experience there are no software tools that will help you. Software gets in the way. The more features you have, the less easy it is to concentrate on the simple act of writing. Ideally you want a free path between brain and page. Try not to clog that path.

If for you typing is easy, use a keyboard and minimalist word processor. If you’re more of a talker, use voice recognition software such as Dragon, or the built-in one on your phone.