Who is 'A' in pretty little liars?

-I'm watching the TV show! I've never read the books but I might start reading them cause I'm hooked on the show! But can anyone who's read the books tell me who is 'A' or I think in the US the series is finished? We are on episode 17!A in the books is two people



SPOILER ALIERT**********************************鈥?br>


A1 was mona she found allisons(courtneys) diary read all the secrets got an extra phone and stalked them all! but she was killed trying to kill spencer



A2-- was Alison(the real alison) alison was never killed instead she had an evil twin who was mental and switched places with alison and alison was mad because she had to be in a mental hospital when she wasnt crazy, then she came home for like a week and killed courtney(who was pretending to be alison) CONFUSING RIGHT?!The idea is that the reader (and watcher) are lead to believe it is the dead 'Allison'.|||We don't know yet :) thats the whole mystery part