I saw it in a book store but couldent buy it at the time, and no that i can buy it, ive forgotten the name!

Its supposedly a VERY long series, by a female author about a girl who was trained from birth to be an assasin, and she might have superhuman powers? I know the book is based in the future, and i think its kind of sci-fi too. I know its not much to go on, but maybe you know what im talking about??? I think the author is still writing the series too, its REALLY LONG. thanks!!