But...
Do take the time now to learn about styles if you are writing a book!
Doing this will save you untold time and angst.
Importance of Styles in Word

Adding to that comment. If you are writing a book using Word, you owe it to yourself to learn more about how to use this very powerful program so that it helps you rather than blocks you. Basic Concepts of Microsoft Word - from Shauna Kelly
Word is NOT a simple program and it is not intuitive.