The board of supervisors of any county in which a county free library has been established may enter into a contract with the board of supervisors of any other county to secure to the residents of the other county such privileges of the county free library as are agreed upon by the contract and upon such considerations as are agreed upon in the contract to be paid into the county free library fund. Thereupon the inhabitants of the other county shall have such privileges of the county free library as are agreed upon by the contract.