How to reference to an other excell book to get the value

[noyau]

The following logic exist to form an archive of the changing values of A1
and A2 [.Cells(xlr, 8) = .Range("A1")]. How the term [.Range("A1")] will
change if the cell A1 is from an other excel book (a "book1" in the desktop)?

Sub ArchiveA2()
Dim xlr As Long
With Sheets("Sheet1")
.Range("G1") = "Value"
xlr = .Cells(Rows.Count, "G").End(xlUp).Row + 1
.Cells(xlr, 7) = Format(Date, "dd/mm/yyyy")
.Cells(xlr, 8) = .Range("A1")
.Cells(xlr, 9) = .Range("A2")
End With
End Sub