The board of directors of any urban school district, in addition to other taxes it is authorized to levy for school purposes or library purposes, may levy a tax on all tangible taxable property subject to its taxing powers for the purchase of sites and for the erection, furnishing and maintenance of public library buildings and reading rooms, when authorized by approving vote of two-thirds of the voters of the district voting on the proposal as submitted by the board. The rate of the levy shall not exceed one and one-half cents on the one hundred dollars assessed valuation, and shall be submitted to be in effect annually for a period not in excess of five years, but nothing herein shall prevent the submission and approval of successive proposals for periods not in excess of five years each.