public function getSheets()
    {
        $this->throwIfBookIsNotAvailable();

        $externalSheets = [];
        $worksheets = $this->getWorkbook()->getWorksheets();

        /** @var Common\Internal\WorksheetInterface $worksheet */
        foreach ($worksheets as $worksheet) {
            $externalSheets[] = $worksheet->getExternalSheet();
        }

        return $externalSheets;
    }