Woman No. 17 by Edan Lepucki

Welcome to the slightly confusing glittery world of the well off in Los Angeles. A woman advertises for a nanny to watch over her toddler son and also keep an eye on her teenage mute son so she can write a memoir about her relationship with the older son. "S" applies and is instantly approved and brought into the family fold without even so much as a background check. What they don't know is that "S" is working on a strange secret art project and it will set the family dynamic topsy-turvy. Except for the precocious toddler, everyone is hurting and keeping secrets. Mute Seth communicates with complete clarity via Twitter as opposed to "S" who seems to lie at every opportunity. No relationship is working from husband and wife to mother and daughter/son. Soon "S" knows every little secret about the family and they know not nearly enough about her.