Three cases, all surveillance operations, start out routinely enough but prove anything but in Lutz’s engaging fifth novel featuring San Francisco PI Isabel “Izzy” Spellman and her eccentric family of investigators (after 2010’s The Spellmans Strike Again). A wife wants her husband followed; a brother wants his sister observed; and parents hire the Spellmans to keep tabs on their Berkeley freshman daughter. Things soon become more complicated as unexpected connections crop up among the initially unrelated cases. Meanwhile, siblings Rae and David are at war over an incident neither will discuss; mother Olivia is overbooked with strange new hobby classes; father Albert makes a major decision without informing his children; and Izzy, loathe to discuss her personal life, refuses to define her relationship with boyfriend Henry Stone. Lutz’s dry, biting humor is in full force, yet there’s more than a hint of melancholy to be found in Izzy’s increasingly solitary pursuits. Agent: Stephanie Kip Rostan, Levine Greenberg. (Mar.)