/// <summary>
 /// Adds the book.
 /// </summary>
 /// <param name="book">The book.</param>
 public static void AddBook(Book book)
 {
     Book currentBook = books.FirstOrDefault(b => b.ID == book.ID);
     if (books.FirstOrDefault(b => b.ID == book.ID) == null)
     {
         books.Add(book);
     }
 }
 /// <summary>
 /// Updates the book.
 /// </summary>
 /// <param name="id">The id.</param>
 /// <param name="book">The book.</param>
 public static void UpdateBook(string id, Book book)
 {
     Book bookToUpdate = books.FirstOrDefault(b => b.ID == id);
     if (bookToUpdate != null)
     {
         book.ID = id;
         books.Remove(bookToUpdate);
         books.Add(book);
     }
 }
 /// <summary>
 /// Updates the book.
 /// </summary>
 /// <param name="id">The id.</param>
 /// <param name="book"></param>
 /// <returns></returns>
 public Book UpdateBook(string id, Book book)
 {
     BookRepository.UpdateBook(id, book);
     return book;
 }
 /// <summary>
 /// Creates the book.
 /// </summary>
 /// <param name="book">The book.</param>
 /// <returns></returns>
 public Book CreateBook(Book book)
 {
     BookRepository.AddBook(book);
     return book;
 }