"Blue Monday" by Nicci French is a psychological thriller that introduces psychotherapist Frieda Klein. When a five-year-old boy goes missing, Frieda is drawn into the investigation by her friend and detective, Karlsson. As she delves deeper into the case, she uncovers unsettling connections to her own past and dark secrets that threaten to unravel her carefully structured life. The novel weaves together suspense, intricate character relationships, and a chilling atmosphere, keeping readers gripped until the final twist.