Flawless (Pretty Little Liars, Book 2) by Sara Shepard

Flawless, the second book in the Pretty Little Liars series by Sara Shepard, continues the story of four girls in Rosewood, Pennsylvania, who are tormented by an anonymous stalker named “A” who threatens to expose their deepest and most scandalous secrets. The girls, Spencer, Aria, Emily, and Hanna, are grappling with the murder investigation of their former best friend, Alison DiLaurentis, and the re-emergence of the mysterious “A” who seems to know everything Alison knew.