How to change the default icon of Custom Post Type

// Add new post type for Book Lessons
add_action('init', 'Book_Lessons_init');
function Book_Lessons_init()
{
	$book_labels = array(
		'name' => _x('Book Lessons', 'post type general name'),
		'singular_name' => _x('Book Lessons', 'post type singular name'),
		'all_items' => __('All Book Lessons'),
		'add_new' => _x('Add new Book Lessons', 'Book Lessons'),
		'add_new_item' => __('Add new Book Lessons'),
		'edit_item' => __('Edit Book Lessons'),
		'new_item' => __('New Book Lessons'),
		'view_item' => __('View Book Lessons'),
		'search_items' => __('Search in Book Lessons'),
		'not_found' =>  __('No Book Lessons found'),
		'not_found_in_trash' => __('No Book Lessons found in trash'),
		'parent_item_colon' => '',

	);
	$args = array(
		'labels' => $book_labels,
		'public' => true,
		'publicly_queryable' => true,
		'show_ui' => true,
		'query_var' => true,
		'rewrite' => true,
		'capability_type' => 'post',
		'hierarchical' => false,
		'menu_position' => 5,
		'supports' => array('title','editor','author','thumbnail','excerpt','comments','custom-fields'),
		'has_archive' => 'Book'
	);
	register_post_type('book',$args);
}

This is the custom post type I am trying to create, can you tell me how do I change the default icon in the admin panel ?