Eager Loading throws error: Trying to get property of non-object

I've posted the question here on stack overflow, but basically, I'm having an issue where eager loading isn't able to correctly associate the keys across models (I think)

All my code is on Stack Overflow, but any help is appreciated. Here's all my code

Here's my 2 migration files

create_authors_table

<?php

# database/migrations/1970_01_01_000001_create_authors_table

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateAuthorsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('authors', function (Blueprint $table) {
            $table->string('author_id');
            $table->string('first_name');
            $table->string('last_name');
            $table->timestamps();

            $table->primary('author_id');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('authors');
    }
}

create_books_table

<?php

# database/migrations/1970_01_01_000002_create_books_table

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateBooksTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('books', function (Blueprint $table) {
            $table->string('book_id');
            $table->string('title');
            $table->string('author_id');
            $table->timestamps();

            $table->foreign('author_id')->references('author_id')->on('authors');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('books');
    }
}

Next, here's my routes

<?php

# routes/web.php
Route::get('/books', 'BookController@index');

** My 2 models**

Author.php

<?php

# app/Models/Author.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Author extends Model
{
    protected $primaryKey = 'author_id';

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

<?php

# app/Models/Book.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Book extends Model
{
    protected $primaryKey = 'book_id';

    public function author()
    {
        return $this->belongsTo('App\Models\Author', 'author_id');
    }
}

Here's my BookController

<?php

# app/Http/Controllers/BookController.php

namespace App\Http\Controllers;

use App\Services\BookService;
use Illuminate\Http\Request;
use App\Models\Book;

class BookController extends Controller
{
    public function index()
    {
        $books = Book::with('author')->get();
        

        $results = [];
        foreach ($books as $book) {
            $result = new \stdClass();
            $result->title = $book->title;
            $result->author_first_name = $book->author->first_name;
            $result->author_last_name = $book->author->last_name;
            $results[] = $result;
        }

        dd($results);
    }
}

Here's some test data to fill the database with:

insert into authors (author_id, first_name, last_name, created_at) 
values 
(
  'ceefb81f-48bb-ff07-1002-27fd8b7272b4', 'Charles', 'Coal', '2018-02-11 00:00:00'
);

INSERT INTO books (book_id, title, author_id, created_at)
VALUES 
(
  'a4e58f2a-36f5-f2fb-960c-c60e2689b337', 'Killing Coal', 
  'ceefb81f-48bb-ff07-1002-27fd8b7272b4',
  '2018-02-11 00:00:00'
),
(
  'a4e58f2a-36f5-f2fb-960c-c60e2689b338', 'Cleaning Coal', 
  'ceefb81f-48bb-ff07-1002-27fd8b7272b4',
  '2018-02-11 00:00:01'
),
(
  'a4e58f2a-36f5-f2fb-960c-c60e2689b339', 'Using Coal', 
  'ceefb81f-48bb-ff07-1002-27fd8b7272b4',
  '2018-02-11 00:00:02'
);

I've turned on logging in MySQL and can see that 2 queries are being run

# turn on logging by running the following in MySQL
SET GLOBAL log_output = "FILE";
SET GLOBAL general_log_file = "/tmp/mysql.logfile.log";
SET GLOBAL general_log = 'ON';

And here's the output of the MySQL log file

2018-02-11T18:52:21.426064Z   262 Prepare   select * from `books`
2018-02-11T18:52:21.426348Z   262 Execute   select * from `books`
2018-02-11T18:52:21.426539Z   262 Close stmt    
2018-02-11T18:52:21.430560Z   262 Prepare   select * from `authors` where `authors`.`author_id` in (?)
2018-02-11T18:52:21.430689Z   262 Execute   select * from `authors` where `authors`.`author_id` in ('ceefb81f-48bb-ff07-1002-27fd8b7272b4')

Any help is appreciated