book.rb

class Book < ActiveRecord::Base
  has_many :taggings, :as => :taggable
  has_many :tags, :through => :taggings
end

movie.rb

class Movie < ActiveRecord::Base
  has_many :taggings, :as => :taggable
  has_many :tags, :through => :taggings
end

tag.rb

class Tag < ActiveRecord::Base
  has_many :taggings, :dependent => :destroy
  has_many :books, :through => :tagings, :source => :taggable, :source_type => "Book"
  has_many :movies, :through => :tagings, :source => :taggable, :source_type => "Movie"
end

tagging.rb

class Tagging < ActiveRecord::Base
  belongs_to :taggable, :polymorphic => true
  belongs_to :tag
end