Wash. Rev. Code § 27.12.180
Instead of establishing or maintaining an independent library, the legislative body of any governmental unit authorized to maintain a library shall have power to contract to receive library service from an existing library, the board of trustees of which shall have reciprocal power to contract to render the service with the consent of the legislative body of its governmental unit. Such a contract shall require that the existing library perform all the functions of a library within the governmental unit wanting service. In like manner a legislative body may contract for library service from a library not owned by a public corporation but maintained for free public use: PROVIDED, That such a library be subject to inspection by the state librarian and be certified by him or her as maintaining a proper standard. Any school district may contract for school library service from any existing library, such service to be paid for from funds available to the school district for library purposes.
RCW 27.12.180