/**
  * Book a current Invoice
  *
  * @param  integer $invoiceNumber
  * @return object
  */
 public function book($invoiceNumber)
 {
     $handle = $this->getHandle($invoiceNumber);
     $number = $this->client->CurrentInvoice_Book(['currentInvoiceHandle' => $handle])->CurrentInvoice_BookResult;
     return $number;
 }