How do I streamline updating a book with one chapter with custom content?

I ​@Barb Binder ‘s reply, but I would lean toward keeping all 18 different .fm chapters (with descriptive names) in the same directory and then swap them out in the book file, using Save As… to create 18 copies of the book file. This way you can do the work once, without having to do file swaps every time you need to update the PDFs.

I would also expect to be able to create one master book file that allows you to navigate and manage all 18 books at once.

EDIT: It was a long day yesterday. I help out on the InDesign and FrameMaker forums, and clearly thought I was answering this on the InDesign forum. However… if you substitute the word “FrameMaker” for “InDesign” my answer still works: 😅

I see two possibilities:

1. Create the 19 location-specific InDesign [FrameMaker] files and swap out the location-specific chapter in the .indd file [.fm file] before exporting the entire book to PDF (repeat for the other location-specific chapters), or
2. Create condition tags, assigned the condition tags to all of the location specific-content and put all of the location-specific information into a single .indd file [.fm file] (that is always included in the book). Before exporting the book to PDF, hide 18 condition tags and show one (for example, show US, hide Canada, Mexico, France, etc). Then repeat for the 18 other locations.
3. For more details on this workflow, see: https://help.adobe.com/en_US/framemaker/using/index.html#t=using-framemaker%2Fuser-guide%2Ffrm_single_sourcing-conditional-text.html

Have you looked into Conditional Text? The downside is you would need to generate the PDF for the entire book 19 times. I don’t know if that takes a lot of time or not. 

Another option is to make the PDFs they way you are and combine them in Acrobat Pro. You can reuse the MAIN section over and over. The downside is any links between the unique chapter and the MAIN section would need to be recreated.