California Code, Health and Safety Code - HSC § 8556

The county recorder of the county in which a map or plat is filed shall index the map or plat in the general index giving reference to date of filing and number or to book and page so that it may easily be found.  The recorder may bind the maps or plats in special books or in his books of maps of subdivisions.  The fee for filing and indexing said map or plat shall be the same as provided for subdivided land under Section 27372 of the Government Code.