_Application.Workbooks property

Returns a Workbooks collection that represents all the open workbooks.

Namespace:  Microsoft.Office.Interop.Excel
Assembly:  Microsoft.Office.Interop.Excel (in Microsoft.Office.Interop.Excel.dll)

Workbooks Workbooks { get; }

The collection returned by the Workbooks property doesn’t include open add-ins, which are a special kind of hidden workbook. You can, however, return a single open add-in if you know the file name. For example, Workbooks("Oscar.xla") will return the open add-in named "Oscar.xla" as a Workbook object.