DCI yet another approach

gistfile1.rb

class User < ActiveRecord::Base
end

class Book < ActiveRecord::Base
end

module CustomerRole
end

class AddToCartContext
  attr_reader :customer, :book

  class Customer < User
    include CustomerRole
  end

  def self.call(customer_id, book_id)
    AddToCartContext.new(customer_id, book_id).call
  end

  def initialize(customer_id, book_id)
    @customer = Customer.find(customer_id)
    @book = Book.find(book_id)
  end

  def call
    @customer.add_to_cart(@book)
  end
end

Interesting, so you would use Customer.find(id) instead of User.find(id).extend(CustomerRole)?

Yes, you are right!
From one side we are using light persistence models,
from another side we can collect role's related code to modules,
and combine it at context for interaction