Thanks Dave + others,

"How would you ever know which workbook is workbooks(1) and which is
workbooks(2)? " was a good question / point.

I was also copying the wrong cells. The working code is now:

Sub Copy_Cell_from_2ndbook_to_1st_book()

' Copy 2nd book's B2 (2,2) to 1st book's E2 (2,5)
Workbooks("Book1.xls").Worksheets("sheet1").Cells( 2, 5).Value = _
Workbooks("counts.csv").Worksheets("summary").Cell s(2, 2).Value

MsgBox "Done"

End Sub