Wanted Pretty Little Liars by Sara Shepard

Wanted is the eighth book of Pretty Little Liars, a series of young adult novels by Sara Shepard.
In Rosewood, majestic estates sprawl for acres, and Tiffany toggle bracelets dangle from every girl's wrist. But not all that glitters is gold, and the town harbors secrets darker than anyone could imagine—like the truth about what really happened the night Alison DiLaurentis went missing...
Back in middle school, Ali plucked Emily, Hanna, Aria, and Spencer from obscurity and turned them into the beautiful, popular girls everyone wanted to be. Ali was the best friend they ever had. But she also made them do terrible things and taunted them with their worst secrets. Now, three years later, all their questions about Ali have finally been answered and they can put this awful chapter of their lives behind them. Or so they think.
In the dramatic conclusion of Sara Shepard's bestselling Pretty Little Liars series, Emily, Hanna, Aria, and Spencer could get everything they've ever wanted—unless A has one more horrifying twist in store.
With Alison's killer on trial, the girls think they can relax. They're shocked to learn of the existence of Alison's twin, Courtney, when she's introduced at a press conference. She attempts to resume her sister's life and begins tentative friendships with Ali's old friends, even striking up a romantic relationship with Emily. Courtney later reveals proof that she is the real Alison, explaining that her twin, Courtney, pretended to be her on the night of the murder. Although skeptical at first, the girls slowly begin to trust Alison... (Summary by Wikipedia)