The Burning Girl by Claire Messud (2017)
The girls first meet Anders Shute after Cassie is bitten by a pit bull at a shelter where they volunteer. (Important note: it was her fault, though I'm annoyed that the pits were treated in a stereotypical "they're so dangerous!" way in this book.) Dr. Shute patches her up, but his slightly odd demeanor casts an ominous pall over the situation that extends to his later relationship with Cassie's mother. There's an implication that he may have inappropriate feelings for Cassie and may have deliberately sought her mother for a relationship to be closer to Cassie.
We never quite get a handle on Anders Shute, nor do we on Cassie herself, and that's one of the themes of this short book: you never really know another person. Julia observes that life is theater, and we all play roles. They may change over time, but we always choose what we let others see. To her, what's happening with Cassie is rather a mystery, just as many situations around us - especially when we're young and adults keep a lot from us - remain partially shrouded and unfathomable.
In keeping with the rather dark tone of the book, Julia and Cassie spent a lot of time visiting an abandoned mental asylum at the height of their friendship. (As one does - in my pre-teen years it was an abandoned house with lots of interesting stuff left behind from the previous inhabitants.) One of my favorite passages in the book was Julia speculating about what happened to the asylum's residents:
"In twenty years, they couldn't all have died - but even if they had, the world wasn't getting any less crazy. So the dying generation of crazies was being replaced all the time by new crazies, a rolling population of lunatics as constant as the tides. Unless it wasn't individuals that changed but society itself: they changed the laws, they closed the asylums, and suddenly the crazies weren't crazy anymore. Maybe when society changed it was decided, somehow, that they never had been crazy; it had all been a category mistake....That would mean you couldn't be sure about things. Better to believe that sane people were sane and crazy people were crazy and you could put the types of people on opposites sides of a wall and keep them separate, clean and tidy. Without that, where did the lunatics go? Where had they gone? Were they among us? Were they us?"
The reviews of this novel seem mixed, and the average rating on Goodreads rather low, but I quite liked it. It was dark, sad, and ominous, and Messud's prose was a pleasure to read. Her only other book I've read was The Woman Upstairs, and I like this one better but stylistically they're comparable. If you liked The Woman Upstairs, I would recommend trying The Burning Girl.