Turtles All the Way Down by John Green
Sixteen-year-old Aza is trying to be a good daughter, a good friend and a good student. She's also trying to track down fugitive billionaire Russell Pickett, who has vanished without a trace. Alongside her Best and Most Fearless Friend, Daisy, Aza sets off in pursuit of the truth - and the $100,000 reward.
The trail leads them to Pickett's son, Davis, a sometime friend of Aza's who might have some of the clues they need. But as her compulsive thought spirals take an ever-tightening grip on her mind, Aza finds herself struggling to keep her investigations - and her life - from falling apart.