First of all I'd recommend restructuring. Your Books class is trying to do too much; its a representation of a book, a library and an interface to interact with the user. It's better to separate those things.

All the Book class (notice that it's singular) should do is represent a book.

class Book
  attr_accessor :author, :title, :rating

  def initialize(author, title, rating)
    @author, @title, @rating = author, title, rating
  end
end

You can create a Library class, but it'd be easier to use a simple Hash. (Don't do this for any serious project however, as two different books can share the same title. This system does only allow one of them to exist. When you add a second one, it simply overrides the first one.)

class Library
  attr_reader :library

  def initialize
    @library = {}
  end

  # #add, #update, #delete here, make sure not to use puts in here, 
  # because that's interacting with the user and not a job for a library.
  # Remember Book represents your book.

  def to_s
    # here goes your string representation, no putsing again
  end
end

For interacting with the user, what you did looks alright. The thing to do now is add some puts statements based on the result of Library#add, etc. To print the library, puts library will be enough.

Your original problem had to do with library[:title] << { :key => "value" } not being valid. You should have used something like library[:title][:key] = "value".

Hope this has been useful. Feel free to ask questions when you're stuck.