Who's A in Pretty Little Liars?

I'm watching the 1st episode of the second season of PLL. I can't stand it anymore, who's A? If you know because you read the book series. Have a hunch, theory, or anything, please tell! :-D Thanks. I once suspected Jenna wasn't really blind, and that the therapist for the girls was A. Please tell me, will pick best answer. Thanks :-D