Cal. Educ. Code § 19103

After the establishment of a county free library, the board of trustees, common council, or other legislative body of any city in the county maintaining a free public library, or the board of trustees of any library district maintaining a district library, may notify the board of supervisors that the city or library district desires to become a part of the county free library system. Thereafter the city or library district shall be a part of the system and its inhabitants shall be entitled to the benefits of the county free library, and the property within the city or library district shall be liable to taxes levied for county free library purposes.