Moving data between two workbook with one a variable name

Hello. I am trying to marry up MLS and tax data. I have the formulas to do that based on a common denominator, but the data sets are in two workbooks. The MLS data is always in the same named wb, but the tax data can be one of many open books. The parts of the process that I don't know how to do are: to read the name of the appropriate tax book from a cell (that I select from a drop down) and then activate that wb (to extract certain records). Thanks ahead of time.