You should update the numbering in the Book palette menu.

– Thanks!!

(I was thinking that gets updated automatically … )

You’re welcome. Unfortunately it doesn’t get updated automatically, and maybe there is a reason for that other than poor implementation. Anyway at least it warns you when you’re exporting it.

maybe there is a reason for that other than poor implementation

Surely. Book feature can be used to achieve a number of multiple document changes.

For example: you can use it to harmonize styles between several docs, you can build a book just to print, package or preflight dozens of separate files at once, you can synchronize colours, conditional texts and variables or you can just export a group of individual files to a single PDF. In all these situations, page number update may be undesired.

So, the way Book works is actually adequate to the many goals it can afford.