Sub MergeDocs()     
     Dim rng As Range     
     Dim MainDoc As Document      
     Dim strFile As String
     Const strFolder = "C:\Book\Chapters\" 'change  to suit
     Set MainDoc = Documents.Add
     strFile = Dir$(strFolder  & "*.doc") ' can change to .docx
      Do Until strFile = ""
          Set rng = MainDoc.Range
         rng.Collapse wdCollapseEnd
          rng.InsertFile strFolder & strFile
         strFile = Dir$()     
     Loop  
End Sub