Unwanted URL with Custom Post Type

function rt_custom_post() {
  register_post_type( 'book',
    array(
    'labels' => array(
        'name' => __( 'Books' ),
        'singular_name' => __( 'Book' ), 
      ),
    'public' => true,
    'has_archive' => false, //this will eliminate the archive page
    'supports' => array('title','editor','thumbnail'),
    'publicly_queryable'  => true
    )
  );

if you add :

'has_archive' => false,

to your register post type, it will remove the archive page. Remember to flush the permalinks by visiting the settings -> permalinks page after you make the change.

Make sure you didn’t create a page called book as well.