Gathering Data from Another Workbook
 

I'm starting a new project where I have two workbooks, one for last year and
one for this year where sales figures are kept by period.
Sheets for each location are named REV001, REV002, etc and are the same name
in each workbook, although this year's workbook has more sheets as there are
more locations.
There are 24 capture periods (2 per month) on the same rows and columns with
data entered for the appropriate period, although the dates of the periods
are different year to year. (So I think a physical cell reference would be
appropriate).
I need to compare year over year sales both for the individual periods but
cumulatively as well and get the sum whether they are ahead or behind, by
each location/worksheet.
Some locations that were new last year do not have sales for all periods.
They may have started in period 10 (May 31, 2005) and have sales to the end
of the year, but do not have sales for periods 1-9 in 2005.
If I were doing a report for the end of August for example, I need to
compare year to year sales against last year, BUT I can't use sales for
periods 1 to 9 in 2006, as there were no sales in those periods last year.
For those periods where there were sales in both years, I would like the sum,
and then a sum of just those periods where there were sales in both years.
I'm not sure the best way to approach this and would appreciate some help.
The unused rows in the 2005 workbook are hidden, but not password protected.

Thanks in advance!