It must function.
You aren't opening the Workbook in the newly generated instance, though. The With-clause instructs VBA to utilise the object for all members that are preceded by a dot, in your case the new Excel Instance. Thus, to access the Workbook in the just-created instance, use
If Dir(bookPath) = "" Then
MsgBox "Planilha não encontrada"
If IsWorkBookOpen(bookPath) Then
Resultado = MsgBox("Arquivo em Uso, Abrir somente leitura?", vbYesNo + vbInformation, "AQUAS")
If Resultado <> vbYes Then Exit Sub
.Workbooks.Open bookPath, True, True
.Workbooks.Open bookPath, True
.Application.WindowState = xlMaximized
Be careful, your code will not be able to close that Excel instance.