Reference another workbook in a formula

I am having trouble inserting a cell reference into a formula. I have a spreadseet that uses data from another workbook. The workbook name changes monthly, so i'd like to be able to place the workbook name in a cell and have the formulas in the spreadsheet use that cell to get the name of the workbook to get the data from. So:

if, for eg, the formula in my spreadsheet is ='[test data file.xlsx]Sheet1'!$B$4+'[test data file.xlsx]Sheet1'!$B$6

I would like to be able to enter test dta file.xlsx into cell A2 of my spreadsheet and replace a reference to A2 into the formulas.
I've tried several ways, but can't seem to get it to work.
Any suggestions?