Let me just start of by saying, this is a fantastic read! If you've read all the Pretty Little Liar's books to date, then there aren't any spoilers But, you need to have read them all.
This is the back story of Allison and Courtney, twins. Both girls were best of friends when they were younger. Something happened to drive them apart. One of the twins overtakes the others identity, and becomes them and live their life as them. However, as life proves, nothing really goes according to plan. Courtney lives Alison's life as Alison. To make sure things go off without a hitch, Courtney dumps all of Ali's friends, and strike up friendships with Aria, Spencer, Hanna and Emily. All of the girls think that this is the real Ali, and will do anything for her. Courtney has it all figured out, well, most of it. She didn't plan on one thing happening. Someone knows what Courtney did, and won't let her get away with it.
What I really liked about this is that at times Courtney was a sympathetic character, and at times we root against her. We get to see Courtney having doubts at what she is doing, and how she is treating Aria, Spencer, Hanna and Emily. Then at other times, she rules her friends with an iron first. She knows all of their secrets, and uses them as weapons against the Pretty Little Liars and controls them. She actually gets offend whey they don't tell her things. She feels she entitled to know everything about them. It really was interesting.
We also see the same scenes in the past books, but from a different point of view. There are also somethings in this book that clears up some of the factors in previous books. I don't' want to go into detail, because I don't want to spoil anything. Let's just say that it was an interesting read. Some events are explained and some new events are introduced.
If you have read all of the PLL books, then this is must read for you. It adds such an interesting element to the series. If you start this, I can promise you won't be sorry. I know I wasn't.
The next book in the PLL series comes out June 4, 2013, and I so can't wait.