Who is "A" on Pretty Little Liars?

Well, I've only started to read the books but according to a friend of mine 'A' is the work of both Mona and Jenna. Remember at the start of the series with all those flashbacks and there was one when Mona was following Aria and Ali before she was popular? If my friend is right then I don't see why ABC would change, that's just stupid. Although, CW has done the same with the Vampire Diaries. They make so many different changes from the books I can barely keep up!