The county legislative body may establish a county library board consisting of seven (7), nine (9) or eleven (11) members. Not more than one (1) county official shall serve on this board. The members shall serve without salary, at least three (3) for one (1) year, two (2) for two (2) years, and two (2) for three (3) years. If the board expands to more than seven (7) members, the additional members shall be appointed by the county legislative body to terms of one (1), two (2) or three (3) years. All successors shall serve for terms of three (3) years. Board members may serve two (2) consecutive terms and may be reappointed after a minimum three-year break in service. Joint library boards with one or more other counties or municipalities may be formed by agreement of the governing bodies of the participating local governments. The members of such joint boards are appointed by the governing bodies of the participating local governments in accordance with the ratio of population in each participating municipality and in the county outside the participating municipality or alternatively, according to an contract providing otherwise or a private act. Counties and cities with populations over four hundred thousand (400,000) may, by 2/3 majority vote, vest supervisory authority over the public library system with the mayor. T.C.A. § 10-3-103. The library board directs the affairs of the library system, including the appointment of a library administrator. The library administrator directs the internal affairs of the library, including hiring and directing such assistants as may be necessary. T.C.A. § 10-3-104.