Is there a way to refer to use a variable as the sheet or file name in an
excel formula?

For example, let's say that in columns A and B, I've got the names of a set
of workbooks and sheets that I want to refer to...

Column A Column B
Book1 Sheet1
Book1 Sheet2
Book1 Sheet3
Book2 Sheet1
Book2 Sheet2
Book2 Sheet3
Book3 Sheet1
Book3 Sheet2
Book3 Sheet3

And let's say I want to know what's in cell A1 in each of those sheets...
So in cell C1, I've got
=[Book1.xls]Sheet1!$A$1

Is there anything I can do short of writing a macro (which is what I'm doing
now) to just get Excel to copy a similar formula into all the rows, where
it's interpreted as
=[{workbook name in column A}]{Sheet name in column B}!$A$1