If you’re knowledgeable about the concept of “styles” for design, and you have a Word document that is styled with the “styles” menu, not the formatting menu, you may wish to try Pressbooks’ one-click importer. To do so,
1. Style your chapter titles as Heading Style 1s in the Word document.
2. Click on Tools -> Import and Select DOCX as your file type.
Note that if your book is not properly styled, Pressbooks may not recognize chapters and you may need to do some cleanup after importing.
For more in-depth instruction on how to import your work from Word, check out http://guide.pressbooks.com/chapter/import-from-word-docx/
To see other import options, go to https://guide.pressbooks.com/chapter/how-to-get-your-book-into-pressbooks/