public function includeBooks(Author $author, ParamBag $paramBag = null)
    {
        $transformer = new BookTransformer($paramBag);

        $books = $author->books()
            ->limit($transformer->getLimit())
            ->offset($transformer->getOffset())
            ->orderBy($transformer->getSortKey(), $transformer->getSortDirection())
            ->get();

        return $this->collection($books, new BookTransformer);
    }