Flawless Pretty Little Liars by Sara Shepard

Flawless is the second book of Pretty Little Liars, a series of young adult novels by Sara Shepard.
In the aftermath of Pretty Little Liars, Spencer, Hanna, Aria, and Emily are more desperate than ever to discover the identity of "A", who is still taunting them with the threat of exposing their many secrets. Their personal problems continue; Aria begins a relationship with Sean, Hanna's ex, and faces the problem of her dad pulling away from the family while continuing his affair with his student, Meredith. At the same time, her little brother, Mike, also finds out. Emily ends things with Maya, and begins loving a boy named Toby Cavanaugh. Hanna begins to have many problems with her best friend, Mona, and deals with stress brought on by her failing body image, the loss of Sean, and the re-appearance of her father in her life. And Spencer continues her secret relationship with her sister's ex boyfriend, Wren. The girls are confused and on edge after Alison's funeral, anxious to stop "A" and keep their secrets buried. "A" taunts Emily about her sexuality and growing feelings for Maya. Hanna is pressured by "A" to tell Naomi and Riley about her eating disorders and is shocked to discover her ex-boyfriend Sean is now pursuing a relationship with Aria. Aria, who is still seeing Ezra, is threatened by "A" to either get rid of Meredith or reveal her father's affair. Spencer plagiarizes one of Melissa's essay for a class, and "A" subsequently reveals to Melissa that Spencer has been secretly dating Wren, Melissa's ex-boyfriend... (Summary by Wikipedia)