Just to add to Bob's reply...
I like to open both workbooks.
Then start typing the formula with an equal sign.
=
Then go to the other workbook, other worksheet and click on the cell that I need
to return.
Then excel will build the formula.
Like Bob wrote, it'll look something like:
=[Book1.xls]Sheet1!$A$1
But if that cell is empty, this formula returns a 0.
So I'd modify it to:
=IF([Book1.xls]Sheet1!$A$1="","",[Book1.xls]Sheet1!$A$1)
And when I close book1.xls, xl will do all the drive/folder stuff for me.
=IF('C:\My Documents\excel\[Book1.xls]Sheet1'!$A$1="","",
'C:\My Documents\excel\[Book1.xls]Sheet1'!$A$1)
(all one cell)
Let excel do the heavy lifting so you don't have to remember any of the syntax.