🚀 go-pugleaf

I assume you somewhere on the body pages in the other book,
probably on a title cover page, have the complete title of the
book, using the variables. Then point that part of the cross-ref
to that particular paragraph. This means that you must break
the cross-ref into two parts, one that refers to the heading or
page of interest, and one that refers to the title page.

Cross-refs cannot pick up text from background text frames,
those used for page headers and footers. Cross-refs can only
pick up text from body page text frames.

There are ways around this limitation. You could, for instance,
in the beginning of each file include a small invisible body page
paragraph, that contains the variables in question, in effect
duplicating the page header. The cross-ref could then refer
to this invisible paragraph by using <$paratext[special_tag]>.

Another less automatic and error-prone solution would be to
define a set of variables that contain all the names of the
publications for your product. You include these variables in
all files in all books, and maintain them in a single FM file
(a template file if you use that). Instead of using a cross-ref
to the other book for the title, you insert the corresponding
variable. Of course, there is a maintenance aspect if you
change the title of a manual, or add new manuals.

-- 
/Thomas Michanek