Using Ryan's formula as a starting point:
=[Book1.xls]Sheet2!$B$51
This is the function you want to try:
="'["&a1&"]"&a2&"'!$B$51"
With Book1.xls in A1
and sheet2 in A2
(The apostrophes aren't always necessary, but they won't hurt if you don't need
them.)
But this won't work. It only builds a string that looks like a formula.
If that sending file is open in the same instance of excel, then you could use:
=indirect("'["&a1&"]"&a2&"'!$b$51")
But as soon as you close that sending workbook (and after excel recalculates),
you'll get an error.
=indirect() needs the sending workbook open.
But...
Laurent Longre has an addin (morefunc.xll) at:
http://xcell05.free.fr/
or
http://xcell05.free.fr/morefunc/english/index.htm
That includes =indirect.ext() that may help you.
===
If you have trouble getting to the site, then search google for indirect.ext.
I found this alternative site:
http://download.cnet.com/Morefunc/3000-2077_4-10423159.html
I didn't look to see if it was the most current version.
I'd check the original site every so often to see if it's working.