Simple REST Client implementation using Retrofit. Interfaces are declared in the GoogleBooksInterfaces file.

RestClient

package com.msvs.bookshelf.retrofit;

import com.msvs.bookshelf.events.BookFoundEvent;
import com.msvs.bookshelf.model.entities.Author;
import com.msvs.bookshelf.model.entities.Book;
import com.msvs.bookshelf.model.entities.Category;
import com.msvs.bookshelf.retrofit.model.Item;
import com.msvs.bookshelf.retrofit.model.QueryResult;

import java.util.ArrayList;
import java.util.List;

import de.greenrobot.event.EventBus;
import retrofit.Callback;
import retrofit.RestAdapter;
import retrofit.RetrofitError;
import retrofit.client.Response;

public class Client {
    private final String API_URL = "https://www.googleapis.com/books/v1";
    private final String ISBN_QUERY_PREFIX = "isbn:";

    private static Client mInstance;

    private Interfaces.GoogleBooksService mService;

    private Client(){
        RestAdapter mRestAdapter = new RestAdapter.Builder()
                .setEndpoint(API_URL)
                .build();

        mService = mRestAdapter.create(Interfaces.GoogleBooksService.class);
    }

    public static Client getInstance() {
        if (mInstance == null) {
            mInstance = new Client();
        }

        return mInstance;
    }

    public void getBookByIsbn(String isbn) {
        Callback<QueryResult> cb = new Callback<QueryResult>() {
            @Override
            public void success(QueryResult result, Response response) {
                //create objects from the query result
                Item queryResult = result.getItems().get(0);

                Book book = new Book(queryResult);

                List<Author> authors = new ArrayList<Author>();
                for (String author : queryResult.getVolumeInfo().getAuthors()) {
                    authors.add(new Author(author));
                }

                List<Category> categories = new ArrayList<Category>();
                for (String category : queryResult.getVolumeInfo().getCategories()) {
                    categories.add(new Category(category));
                }

                EventBus.getDefault().post(new BookFoundEvent(book, authors, categories));
            }

            @Override
            public void failure(RetrofitError error) {
            }
        };

        mService.getBookByIsbn(ISBN_QUERY_PREFIX + isbn, cb);
    }
}