Sum cells from multiple files in same folder

Private Function GetValue(path, file, sheet, ref)
    Dim arg As String
    If Right(path, 1) <> "\" Then path = path & "\"
    arg = "'" & path & "[" & file & "]" & sheet & "'!" & _
      Range(ref).Range("A1").Address(, , xlR1C1)
    GetValue = ExecuteExcel4Macro(arg)
End Function

Sub LoopThruBooks()
    Dim p, f, s, a, r
    p = "C:\excelfolder\"
    f = Dir(p & "*.xls")
    Do While f <> ""
        s = "Sheet1"
        a = "A1"
        r = r + 1
        Range("A" & r) = GetValue(p, f, s, a)
        f = Dir()
    Loop
End Sub

Sub LoopThruBooks()
    Dim p, f, s, a, r
    p = "C:\excelfolder\"
    f = Dir(p & "*.xls")
    s = "Sheet1"
    a = "A1"
    Do While f <> ""
        r = r + 1
        Range("A" & r) = GetValue(p, f, s, a)
        f = Dir()
    Loop
End Sub

Sub DirFiles()
    Dim FileName As String, FileSpec As String, FileFolder As String
    Dim wb As Workbook
    Dim dblSum As Double, fn As String
    
    dblSum = 0
     
    FileFolder = ThisWorkbook.path
    FileSpec = FileFolder & "\test*.*"
     
    FileName = Dir(FileSpec)
    If FileName = "" Then Exit Sub
     
     '   Loop until no more matching files are found
    Do While FileName <> ""
        If IsWorkbookOpen(FileName) = False Then
            'Set wb = Workbooks.Add(FileFolder & FileName)
            'DoEvents
            dblSum = dblSum + (GetInfoFromClosedFile(FileFolder, FileName, "Sheet1", "A1"))
            'wb.Close True
        End If
        FileName = Dir()
    Loop
    
    MsgBox dblSum
End Sub
 
Function IsWorkbookOpen(stName As String) As Boolean
    Dim Wkb As Workbook
    On Error Resume Next ' In Case it isn't Open
    Set Wkb = Workbooks(stName)
    If Not Wkb Is Nothing Then IsWorkbookOpen = True
     'Boolean Function assumed To be False unless Set To True
End Function

'=GetValue("c:\files", "budget.xls", "Sheet1", "A1")
'    wbPath = "d:\files"
'    wbName = "budget.xls"
'    wsName = "Sheet1"
'    cellRef = "A1:R30"
Private Function GetInfoFromClosedFile(ByVal wbPath As String, wbName As String, wsName As String, cellRef As String) As Variant
    Dim arg As String
    GetInfoFromClosedFile = ""
    If Right(wbPath, 1) <> "\" Then wbPath = wbPath & "\"
    If Not FileExists(wbPath & "\" & wbName) Then Exit Function
    arg = "'" & wbPath & "[" & wbName & "]" & wsName & "'!" & Range(cellRef).Address(True, True, xlR1C1)
    On Error Resume Next
    GetInfoFromClosedFile = ExecuteExcel4Macro(arg)
End Function

Function FileExists(sFilename As String) As Boolean
  Dim fso As Object, tf As Boolean
  Set fso = CreateObject("Scripting.FileSystemObject")
  tf = fso.FileExists(sFilename)
  Set fso = Nothing
  FileExists = tf
End Function