A Line in the Dark by Malinda Lo
Jess Wong is Angie Redmond's best friend. And that's the most important thing, even if Angie can't see how Jess truly feels. Being the girl no one quite notices is OK with Jess anyway.
If nobody notices her, she's free to watch everyone else. But when Angie begins to fall for Margot Adams, a girl from the nearby boarding school, Jess can see it coming a mile away. Suddenly her powers of observation are more a curse than a gift.
As Angie drags Jess further into Margot's circle, Jess discovers more than her friend's growing crush. Secrets and cruelty lie just beneath the carefree surface of this world of wealth and privilege, and when they come out, Jess knows Angie won't be able to handle the consequences.
When the inevitable darkness finally descends, Angie will need her best friend.
Paperback / 288 pages