book.rb

class Book
  attr_reader :title

  DONT_CAPITALIZE = %w(and or the an of in a)

  def title=(title)
    @title = title.capitalize.split.map do |word|
      if DONT_CAPITALIZE.include?(word.downcase)
        word
      else
        word.capitalize
      end
    end.join(" ").strip!
  end
end

• Some people don't like following a begin...end block with a method, and opt for braces instead
• Some might use the ternary operator cond ? if_true : if_false, but I opted for the plain ole if...else..end
• @dont_capitalize was really a constant and thus I moved it to DONT_CAPITALIZE