I use a variation of FrameMaker 10's FM_Outputs_CondText.jsx to automate generation of PDFs.
What do I need to add to force a book update?
It looked to me as if it were doing that, but I just noticed that the build date variable in the front matter is updated only when I manually update the book.
The FrameMaker 10 Scripting Guide lists all the objects and methods. For the Book object, there is an UpdateBook method. I guess that is the one you are looking for. It is described on pages 435 and following.
To be a little more elaborate: I checked the script you mentioned and it only updates cross-references when opening a book and its files. That is not the same as updating the book and all its variables. You do need to use the Book.UpdateBook method for that explicitly, after first opening all files in the book. That amounts to the same as manually updating the book.