I have some formulas that refer to other workbooks. I want the formula to
point to a different column, depending on the current month; e.g., in
January, the data will be in column E (and relative row references), in
February, the data will be in column F, and so on.

How do I create the formula with the cell reference as a "Variable" that can
refer to other cells where the correct column identifier is indicated via the
VLOOKUP function, or is just entered?