Register Custom Post Type

custom-post-type-with-publicize.php

function codex_custom_init() {
    $args = array(
      'public' => true,
      'label'  => 'Books',
      'supports' => array ( 
        'title', 
        'editor', 
        'excerpt', 
        'publicize', 
        'thumbnail', 
        'comments', 
        'revisions', 
        'custom-fields', 
        'page-attributes', 
        )
    );
    register_post_type( 'book', $args );
}
add_action( 'init', 'codex_custom_init' );

custom-post-type.php

function codex_custom_init() {
    $args = array(
      'public' => true,
      'label'  => 'Books'
    );
    register_post_type( 'book', $args );
}
add_action( 'init', 'codex_custom_init' );