working with 2 excel workbooks with changing worksheets?

I have a workbook that contains a column that gets its information from cells from a different workbook. I am currenntly using the following to get the data.
=IF(‘[Revenue_Charged_to_3PD.xls]WE 2.25’!$A$15=”Hugh Gordon”,SUM(‘[Revenue_Charged_to_3PD.xls]WE 2.25’!$W$15),”Incorrect Driver Cell”)

The problem is that both workbooks will have another sheet copied and added at the end using the week ending date as its name in the following format WE 2.25 for feb 25.
I would like to automate the formula to look for that new sheet name.