I need help dynamically changing the source of where my data comes from in a VLookup.

I'm working off a document that aggregates data for each month of the year. The data for this comes from several different documents on a monthly basis.

So I need my Vlookup to source from different documents.

So for example, I would have this formula in the September2009 column that needs to reference the September 2009 document.

=VLOOKUP(A5,'SourceData September2009.xls]National - FM'!$B$3:$F$69,2,0)

But the next column needs to source from the October2009 document, so:

=
=VLOOKUP(A5,'SourceData October2009.xls]National - FM'!$B$3:$F$69,2,0)

I feel like this is fairly simple, I just don't know how to do it. Thanks.