How to get average with orderBy Desc in Laravel 5

I have 2 tables in my database.

books and ratings

in books id, name

in ratings

id, book_id, rating

i have set has many relationship for these models.

So in Book model -

 public function ratings()
    {
        return $this->hasMany('App\Rating');
    }

in Rating Model -

  public function book()
    {
        return $this->belongsTo('App\Book');
    }

Now i want to fetch all books with there average rating but order by high rating.

so i high rated books first and then low ratings.

So How i can join 2 tables to achieve this result.

Thanks.

via Chebli Mohamed