Best of all, iA Writer is an app that you can do multiple different types of writing.
You can get a monthly subscription for $4.99 a month, a six-month subscription for $10.99 and an annual subscription for $39.99 a year. It is a free app, but does contain in-app purchases, which is a subscription to the service. The good thing too is that by clicking here, you can get a free trial of the Mac version of the app.
Here are some great book-writing apps for the Mac:
