Since we have a `Book` object, let's just have `@books` be an array that holds that list of `Book` objects for us:

```ruby
class Library
  def add_book(book)
    @books << book
  end
end
```

...which we would then call like so:

```ruby
book = Book.new 'A Tale of Two Cities', 'Charles Dickens'

library.add_book(book)
```