Cross References Across Multiple Books

[kgrace]

I have a situation in which I would like to create several books that
are related.  These books in their own right are going to result in many
files for each book.  I was wondering if there is a way to
cross-reference between these books (output as PDF files) if they are in
different subdirectories i.e., I would like for each book to reside in
its own subdirectory.  I have tried testing the cross-references and
they only seem to work when the files/books all reside under the same
directory.  If this is the case this is going to make for a directory
that is bloated.  Any ideas on being able to make the cross references
work "across" various subdirectories?