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.
Thanks. It turned out the problem was that somebody used the Modification Date system where they should have used the Current Date variable, after I fixed that the script works fine as is.