As stated on the first post, I also recommend the use of Outline Numbering, BUT, as also stated there, this system have its limitations. You can see the second method as a "work-around" to obtain the numbering structure of a literary work.UPDATE to the second method
If your book needs to be separated on "Parts" and then on "chapters", and you need to not
restart the chapter numbers on a new part, note that you can also use a modified version of method 2 above (mixing it with the recommended method), in which you use a numbered style only
for the "Part", and Outline numbering for the chapters. In this way, the chapters will display on the navigator and you will be able to move them if needed. "Parts" will not be present on the navigator, but after all I never see a book with too many of them. You know, even Tolkien used only six parts...