Perfect (Pretty Little Liars Bk3) - Sara Shepard

Spencer, Aria, Emily, Hanna and their best friend Alison were the most popular girls at Rosewood Day School. Alison was the group's ringleader, the one who knew all their darkest secrets. So when Alison vanished one night, Spencer, Aria, Emily, and Hanna's grief was tinged with relief.

But when Ali's body was later discovered in her own backyard, the girls were forced to unearth some ugly memories of their old friend, too. Is there more to Alison's death than anyone realises? Now someone named A, someone who seems to know everything, is pointing the finger for Ali's murder at one of them. And A is poised to ruin their perfect little lives . . . for ever.