AuthorResource.php

<?php

namespace App\Modules\Authors\Resources;

use App\Modules\Books\Resources\BookResource;
use Illuminate\Http\Resources\Json\Resource;

class AuthorResource extends Resource
{
    public function toArray($request)
    {
        return [
            'id' => $this->resource->id,
            'name' => $this->resource->name,
            'books' => $this->whenLoaded('books', function () {
                return BookResource::collection($this->resource->books);
            })
        ];
    }
}