The board of library trustees of a county library district may annually, during the month of May, certify to the board of county commissioners of the county in which such district is situated the amount of money required to maintain and operate the free public library during the ensuing year and the amount of revenue anticipated from all sources other than a tax levy on the taxable property of said district. The board of county commissioners may annually levy a tax on the taxable property of the district not to exceed one mill for the purpose of providing funds for library operation pursuant to said certification. The tax so levied shall be in addition to all other levies authorized by law.