This way you get the .prj (Mif2Go project file) settings for the book, and the output conversion files are updated automatically. Never convert a chapter alone without the book file open in FrameMaker.
If you change only wording in one chapter, just rerun Mif2Go on that chapter, and the entire book will be updated accordingly. However, if you add new cross references that link to other chapters (thus modifying those chapters also), or if you make changes to several chapters, it is better to run Mif2Go on the entire book.
> 32 Automating Mif2Go conversions > 32.3 Converting a single chapter of a book