What happened in the Pretty Little Liars book Flawless by Sara Shepard?

"Don't get too comfortable. It's not over until I say it is." -A

Alison DiLaurentis is out of the picture, but A keeps sending Aria, Emily, Hanna and Spencer suspicious text about things only Ali could know-and that's not the Liars' only problem. Aria is struggling with a secret that could tear her family apart, and toying with a dangerous, irresistible new love.