how to make interface explicit with Hash stores?

I have a class that uses hash as its internal store. What's a good way
to communicate the interface of my methods. It seems like the user would
only know how to use it if there's a comment or a test case. This is the
interface that I want them to use:

  book = Book.new
  book.add_content(1 => some_text)
  book.add_content(2 => some_text)

This is the class that I have:

  class Book
    def initialize()
      @hash = {}
    end

    def add_contents(hash)
      @hash.merge! hash
    end

    def contents
      @hash
    end
  end

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