function WorkBookIndex(WorkBookName: Ansistring): integer;
var
  i, n: integer;
begin
  //проверка на наличие книги с этим именем
  n := 0;
  if VarIsEmpty(Excel) = false then
    for i := 1 to Excel.WorkBooks.Count do
      if Excel.WorkBooks[i].FullName = WorkBookName then
      begin
        n := i;
        break;
      end;
  WorkBookIndex := n;
end;

procedure TForm1.Button11Click(Sender: TObject);
var
    EXWorkSheets : Sheets;
      EXSheet : Variant;
    SheetName : OleVariant;
    i: integer;
begin
TLog.Lines.add(IntToStr( WorkBookIndex('D:\3.xlsx')));
end;