Mike,

I'm not sure I understand what you're trying to do, but maybe this is it:

I created 3 excel files Book1, Book2, Book3.
In Book1 I have a validation list in A1 that contains, Book2,Book3
I have a validation list in B1 that contains A1,B2

In Cell A4 (Book1) I have the following formula:
="["&A1&"]Sheet1!"&B1

Then in Cell A5 I have this formula:
=INDIRECT(A4)

Now, what happens is that the selections in A1 and B1 change the result in A5.

Art