You can't select a sheet in a non-active workbook.
You must first activate the workbook, then you can select the sheet.
workbooks("A").activate
workbooks("A").worksheets("B").select
When you use Activate it automatically activates the workbook.
Note you can select >1 sheet in a workbook:
activeworkbook.sheets(array("sheet1","sheet3")).select
but only one sheet can be Active, and if you activate a sheet which is not part of a multi-sheet selection then those other sheets will become un-selected.