Rails 5.1: retrieve records for nested association with includes

I'm trying to implement includes in has_many / belongs_to association similar to this example:

class Author < ApplicationRecord
  has_many :books, -> { includes :line_items }
end

class Book < ApplicationRecord
  belongs_to :author
  has_many :line_items
end

class LineItem < ApplicationRecord
  belongs_to :book
end

At the moment when I do @author.books I see in my console it loads Book and LineItem and shows records of Book, no records for LineItem. I get undefined method error when I try @author.books.line_items. Nor does @author.line_items work.

How do I get LineItem records for Author, please? Thank you!