class User
{
    // ... books is defined previously
    public function latestBook()
    {
        return $this->books()->one()->latestOfMany();        // Previously        // return $this->hasOne(Book::class, 'author_id')->latestOfMany();     }
}

public function testLatestOfMany()
{
    $user = User::create(['name' => 'John Doe']);
    $book1 = Book::create(['title' => 'Title 1', 'author_id' => $user->_id]);
    $book2 = Book::create(['title' => 'Title 2', 'author_id' => $user->_id]);
    $book3 = Book::create(['title' => 'Title 3', 'author_id' => $user->_id]);

    $latestBook = User::find($user->_id)->latestBook()->get();
    $this->assertEquals($book3->_id, $latestBook->_id);
}