Section 34. The board of trustees of a free public library in any city or town, or in the absence of such board, the city or town official possessing the appointive powers of such board shall, except in the case of those employees subject to the provisions of chapter one hundred and fifty E, execute a written employment contract with an employee of said library outlining the basic conditions of employment, including but not limited to the establishment of a probationary period and the procedure for dismissal during this period and the establishment of a procedure which specifies the cause for dismissal after the completion of such probationary period.