WebAPI2 Unit Testing with IHttpActionResult

BookController.cs

// GET api/books
public IHttpActionResult Get()
{
    return Ok(_repository.GetBooks().AsEnumerable());
}
  
// GET api/books/1
public IHttpActionResult Get(int id)
{
    var book = _repository.GetBook(id);
  
    if (book == null)
    {
        return NotFound();
    }
  
    return Ok(book);
}

// PUT api/books/1
public IHttpActionResult Put(int id, Book book)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }
 
    return Ok(_repository.UpdateBook(book));
}

// POST api/books
public IHttpActionResult Post(Book book)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    var newBook = _repository.AddBook(book);
    return CreatedAtRoute("DefaultApi", new { newBook.Id }, newBook);
}

Test_BookController.cs

[TestMethod]
public void GetShouldReturnAllBooks()
{
    var controller = new BooksController();
    var actionResult = controller.Get();
    var response = actionResult as OkNegotiatedContentResult<IEnumerable<Book>>;
    Assert.IsNotNull(response);
    var books = response.Content;
    Assert.AreEqual(1, books.Count());
}

[TestMethod]
public void GetWithIdItShouldReturnThatBook()
{
    var controller = new BooksController();
    var actionResult = controller.Get(1);
    var response = actionResult as OkNegotiatedContentResult<Book>;
    Assert.IsNotNull(response);
    Assert.AreEqual(1, response.Content.Id);
}

[TestMethod]
public void PutShouldUpdateBook()
{
    var controller = new BooksController();
    var book = new Book { Id = 1, Title = "asp.net webapi2", Author = "scott" };
    var actionResult = controller.Put(book.Id, book);
    var response = actionResult as OkNegotiatedContentResult<Book>;
    Assert.IsNotNull(response);
    var newBook = response.Content;
    Assert.AreEqual(1, newBook.Id);
    Assert.AreEqual("asp.net webapi2", newBook.Title);
    Assert.AreEqual("scott", newBook.Author);
}

[TestMethod]
public void PostShouldAddBook()
{
    var controller = new BooksController();
    var actionResult = controller.Post(new Book
    {
        Title = "asp.net webapi2",
        Author = "scott"
    });
    var response = actionResult as CreatedAtRouteNegotiatedContentResult<Book>;
    Assert.IsNotNull(response);
    Assert.AreEqual("DefaultApi", response.RouteName);
    Assert.AreEqual(response.Content.Id, response.RouteValues["Id"]);
}

I am getting error like "The type APIController is defined in an assembly that is not referenced.Please help me in that