I haven't yet figured out a good strategy for keeping track of the name
of the original workbook after the user has switched to another one.

For instance, in Book1.xls a macro might open Book2.xls, and if so, I
want Book2's macro to re-activate Book1. But since Book1's original
macro was in a menu, it could have been called when who-knows-what
workbook was the active one.

I've currently got the Book1's name hard-coded in Book2's macro, but
that's a time bomb for anyone who later renames Book1. Using
Personal.xls is not feasible with various users on a LAN.

It's easy to use

myOriginalBook = ActiveWorkbook.Name

or whatever, to grab the name of Book1 when the Book1 is opened, but
how can I store this string where it will still be available to Book2's
macro?

I was thinking of putting it in a Static variable in a subroutine that
I call from both workbooks, but I can't call the subroutine from
another workbook unless I spell out which workbook it's in, which means
I have to know the workbook name I'm trying to find out. Same problem
with storing it in a defined Name object in the original workbook, and
probably same problem with using custom document properties (or using
the registry(?) which seems too advanced for me).

Is this as Catch-22 as I'm imagining it to be?