ASP.NET and EF Core Web API returning null for an object within an object

I'm using web api to return the data of a Book stored in an SQlLite database, using EF Core, on ASP.NET Core MVC.

Here is the DbContext code:

using Microsoft.EntityFrameworkCore;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

namespace Yara.SQLite
{
    public class BookingContext : DbContext
    {
        public DbSet<Book> Books { get; set; }
        public DbSet<Author> Authors { get; set; }

        //public DbSet<Author> Authors { get; set; }
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlite("Data Source=books.db");
        }
    }

    public class Book
    {
        public int BookId { get; set; }
        public string Name { get; set; }

        public Author Author { get; set; }
    }

    public class Author
    {
        public string Name { get; set; }

        public int AuthorId { get; set; }
    }
}

and here is the web api controller code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Yara.SQLite;


namespace Yara.Controllers.API
{
    public class UsersController : Controller
    {
        [HttpGet]
        [Route("api/books")]
        public IEnumerable<Book> GetBooks()
        {
            using(var c = new BookingContext())
            {
                return c.Books.ToList();
            }
        }

        [HttpGet]
        [Route("api/books/{id:int}")]
        public Book GetBook(int id)
        {
            using(var c = new BookingContext())
            {
                return c.Books.FirstOrDefault(b => b.BookId == id);
            }
        }

        [HttpPost]
        [Route("api/books")]
        public Book AddBook([FromBody] Book book)
        {
            using(var db = new BookingContext())
            {
                db.Books.Add(book);
                db.SaveChanges();
                return db.Books.Where(o => o == book).FirstOrDefault();
            }
        }
    }
}

Both the GET requests seem to work fine, and so does the POST. When I want to add a new Book, I'll send a POST request, like so:

https://gyazo.com/e96c81479f7ccc084401d70cf13c4cbe

and when i send that request, I'll get my data, like so:

{
    "bookId": 3,
    "name": "A book on US Politics",
    "author": {
        "name": "Enra",
        "authorId": 3
    }
}

When I try to do a GET request on /api/books/3 for example, it'll return this data:

{
    "bookId": 3,
    "name": "A book on US Politics",
    "author": null
}

I've even checked the database using SQLLite browser and the data exists. It doesn't make sense to me.