Section 19. The board of library commissioners may expend such sums as may be appropriated for the extension and encouragement of library services within the commonwealth. The said board is hereby designated as the state agency to deal with the federal government with respect to federal grants which may be made available to the commonwealth for promoting library services, and to administer such state plans as may be approved as a condition of such grants. The board may contract with any other state agency, city or town, public or private library to provide improved library services in an area, or to secure such library services as may be agreed upon, which services may include, but need not be limited to, the lending of books and related library materials, the establishment of branch libraries, depositories or bookmobile service, and to co-operative purchasing and processing of books, recordings, films and related library materials. The board shall cooperate with the Library of Congress or its successors relative to the administration of the program of talking books and may expend such state, federal or other funds as are available for such purpose. The board shall also represent the commonwealth in the receipt and disbursement of funds made available to the commonwealth from any private source for the promotion of library services. The state treasurer shall be the custodian of funds for this purpose received from the federal government or private services.