library.rb

class Shelf
  ...
  def store(@book)
    # logic to find the correct location on itself for @book
  end
end

class Book
  attr_accessor :title, :author
  def initialize(args)
    args = defaults.merge(args)
    @title = args[:title]
    ...
  end
  def defaults
    {title: "Unknown", author: "Unknown"}
  end
  ....
end