code: private void addBook(String isbn, String title, String synopsis, int quantity) {
          Book book = new Book();
          book.setIsbn(isbn);
          book.setTitle(title);
          book.setSynopsis(synopsis);
          isbns_to_books.put(isbn, book);
          isbns_to_quantities.put(isbn, quantity);
      }
docs: Adds the book.
  
  @param isbn the isbn
  @param title the title
  @param synopsis the synopsis
  @param quantity the quantity