Liza Cody: Head Case (UK 1993)
From the Publisher:
Thea Hahn is a highly gifted child, exceptional in mathematics, astrophysics and chess. But Thea has disappeared, and her chilly parents are very worried. They hire Anna Lee to find their 16-year-old daughter. When Thea turns up, however, she seems deranged and is raving about murder.
Liza Cody: Head Case. Arrow Books, ISBN: 0099203219 (January, 1993), 194 p., £3.99.