Every taxpayer under this Act shall keep books, records, papers and other documents which are adequate to reflect the information which such taxpayers are required by Section 3 of this Act to report to the Department by filing monthly returns with the Department. All books and records and other papers and documents required by this Act to be kept shall be kept in the American language and shall, at all times during business hours of the day, be subject to inspection by the Department or its duly authorized agents and employees. Books and records reflecting gross receipts received during any period with respect to which the Department is authorized to establish liability as provided in Sections 4 and 5 of this Act shall be preserved until the expiration of such period unless the Department, in writing, authorizes their destruction or disposal at an earlier date.
     The Department may, upon written authorization of the Director, destroy any returns or any records, papers or memoranda pertaining to such returns upon the expiration of any period covered by such returns with respect to which the Department is authorized to establish liability.