Android - API response returns OK but list is empty

Question

I am using Google Books API to display relevant informations about books (like title, authors, publisher...) in a recyclerview and to do that I am using Retrofit2 everything seems to be working fine the status code is 200 (which means OK according to Google's documentations) but the list that contains the books is empty, below you will find the code for each relevant class attached

Starting with the API interface, that includes the method of search a book by its id:

import retrofit2.Call;
import retrofit2.http.GET;
import retrofit2.http.Path;
import retrofit2.http.Query;

public interface ApiInterface {
    @GET("volumes/{id}")
    Call<BookResponce> getBooks(@Path("id") String id, @Query("API_KEY") String apiKey);
}

Here's the Retrofit instance class that I am using:

import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class RetrofitInstance {
    private static Retrofit retrofit;
    private static final String BASE_URL = "https://www.googleapis.com/books/v1/";

    public static Retrofit getRetrofitInstance(){
        if (retrofit == null){
            retrofit = new retrofit2.Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
        }
        return retrofit;
    }
}

Below is the Book.java Model class

@SerializedName("id")
private String bookId;

@SerializedName("description")
private String bookDescription;

@SerializedName("title")
private String bookTitle;

@SerializedName("authors")
private List<String> bookAuthors = new ArrayList<>();

@SerializedName("averageRating")
private float bookRating;

Now, here's the Book Response class

@SerializedName("kind")
private String bkind;

@SerializedName("items")
private List<Book> results;

@SerializedName("totalItems")
private int totalItems;

Finally, the Main Activity class

public static final String BOOKS_API_KEY = "...";
private List<Book> bookList = new ArrayList<>();
private RecyclerView rv;
private BookAdapter mAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home);

    rv = (RecyclerView) findViewById(R.id.recycler_a);

    mAdapter = new BookAdapter(bookList);

    rv.setLayoutManager(new LinearLayoutManager(this));
    rv.setAdapter(mAdapter);

    DisplayBook();
}

private void DisplayBook(){
    ApiInterface apiService =
            RetrofitInstance.getRetrofitInstance().create(ApiInterface.class);

    retrofit2.Call<BookResponce> call = apiService.getBooks("zyTCAlFPjgYC", BOOKS_API_KEY);
    call.enqueue(new Callback<BookResponce>() {
        @Override
        public void onResponse(retrofit2.Call<BookResponce> call, Response<BookResponce> response) {
            int statusCode = response.code();
            int totalItems = response.body().getTotalItems();
            List<Book> books = response.body().getResults();
            bookList = books;
            mAdapter.notifyDataSetChanged();

            Toast.makeText(getApplicationContext(), "code: "+statusCode+" | total "+totalItems, Toast.LENGTH_LONG).show();
        }

        @Override
        public void onFailure(retrofit2.Call<BookResponce> call, Throwable t) {
            Toast.makeText(getApplicationContext(), "fail", Toast.LENGTH_LONG).show();
        }
    });     
}

Please keep in mind that the main issue here:

STATUS CODE = 200 (Success)
but
TOTAL ITEMS RETURNED = 0 /** List<Book> books = response.body().getResults(); */