Using a DataGrid inside a WPF ComboBox

Hello:
    I'm working on a project which requires the use of a DataGrid table as an item in ComboBox.  It is needed so the users can have a full description of the choices they are about to make.  I have succeeded in adding the DataGrid, populated with data from an XML document, to the ComboBox.  When clicked, the ComboBox displays the table.  The problem I have is in how to "connect" the chosen item in the table to the popup in the ComboBox.

I have set the ComboBox as follows:

      
           
               
           
       

The popup settings inside "ComboBoxStyle3":

                          
                               
                                   
                                       
                                       
                                                                                                     ItemsSource="{Binding Source={StaticResource InventoryData},XPath=/Inventory/Books/Book}">
                                           
                                               
                                           
                                           
                                               
                                               
                                           
                                       
                                   
                               
                           




So what I need is to populate the popup with the first column of the table (if only I could make the popup work).

Thanks in advance,
Pedro