ErrorException in BooksController.php line 19 : Indirect modification of overloaded property App\Models\Book::$author has no effect

Here I'm trying to prevent duplicate values in my DB. But unexpectedly I got this error. Moreover author_id is default null. even I'm unable to throw an exception. Any help that would be a great help for me.

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Input;
use DB;

use App\Models\Book;
use App\Models\Author;

class BooksController extends Controller
{
    public function store(Request $request)
    {
        $input = Input::all();

        $book = new Book;
        $book->id = Input::get('id');
        $book->title = Input::get('title');
        $book->author->id = Input::get('author_id');
        $book->price = Input::get('price');
        $book->isbn = Input::get('isbn');
        $book->language = Input::get('language');
        $book->isbn = Input::get('isbn');
        $book->year_of_publisher = Input::get('year_of_publisher');
        if(DB::table('books')->where('title', '=', Input::get('title'))->get()){
            return 'Book is already there';
        } else{
        $book->save();
        return $book; }
    }
    public function bookFormat($book){  
        return [ 
            'Id' => $book->id,
            'Title' => $book->title,
            'Author' => [ 
                'Id' => $book->author->id,
                'First_name' => $book->author->first_name,
                'Last_name' => $book->author->last_name,
                'Age' => $book->author->age,
                'Country' => $book->author->country
            ],
            'Price' => $book->price,
            'ISBN' => $book->isbn,
            'Language' => $book->language,
            'Year' => $book->year_of_publisher
        ];
    }
}

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