You could be asking about using INDIRECT() ..
Try this example:
Let's open 2 new books, Book1.xls and Book2.xls
In Book1.xls, in Sheet1
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Enter the value "99" in B1
In Book2.xls, in Sheet1
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Enter the following values in cells:
B1: Book1
C1: Sheet1
D1: B1
Put in say B2: =INDIRECT("["&B1&"]"&C1&"!"&D1)
B2 will return "99"
(the value in B1 in Sheet1 of Book1.xls)
The use of INDIRECT() above is functionally equivalent
to having in B2: =[Book1]Sheet1!B1
but with the flexibility of defining in cells B1:C1
the filename, the sheetname, and the cell reference
that is desired, either via manual input
or formula returns in cells B1:C1
Note: The source Book1.xls must be concurrently open
with Book2.xls for INDIRECT() to work