Sub ProcessAttachment(Item As Outlook.MailItem)
Dim olAtt As Attachment
Dim strFilename As String
Dim xlApp As Object
Dim xlWB As Object
Dim xlTarget As Object
Const strTarget As String = "C:\Path\TargetBookName.xlsx"
    If Item.Attachments.Count > 0 Then
        For Each olAtt In Item.Attachments
            If Right(olAtt.Filename, 4) = "xlsx" Then
                strFilename = "C:\Path\" & olAtt.Filename
                olAtt.SaveAsFile strFilename
                Set xlApp = GetObject(, "Excel.Application")
                If Err <> 0 Then
                    Set xlApp = CreateObject("Excel.Application")
                End If
                On Error GoTo 0
                Set xlWB = xlApp.workbooks.Open(strFilename)
                Set xlTarget = xlApp.workbooks.Open(strTarget)
                '+++++++++++++++++++++++++++++
                'Do stuff with the 2 workbooks
                '+++++++++++++++++++++++++++++
                xlWB.Close 0        'Close attachment without saving
                'Kill strFilename 'Optional delete the saved attachment
                Set xlWB = Nothing
                Set xlTarget = Nothing
                Set xlApp = Nothing
                Exit For
            End If
        Next olAtt
    End If
End Sub