book.rb

class Book
  attr_reader :title

  def title=(title)
    @title = ""
    @dont_capitalize = %w(and or the an of in a)
    words = title.split
    words.each_with_index do |word, index|
      if @dont_capitalize.include?(word) && index != 0
        @title += "#{word} "
      else
        @title += "#{word.capitalize} "
      end
    end
    @title.strip!
  end
end

Based on previous feedback now doing interpolation within the Ruby convention.

https://gist.github.com/jcsalterego/5282603