Island of Last Resorts (Marked for Retribution series)

PI Kate Weller's co-workers have been invited to an exclusive island retreat by Julian Frazier, a friend of a wealthy client. Frazier has written his own murder mystery script, and the PIs must work out whodunnit. But they discover that the murder Frazier wants them to solve is a real cold case, and there's a killer twist that isn't in the script.