Displeasure island by Alice Bell

Professional medium turned detective Claire, her best friend Sophie (a 17-year-old ghost) and their pals are enjoying a much-needed cheap holiday in an unfinished hotel on Spike Island off the West Coast of Ireland. Claire is flattered to be asked by the local ghost of a pirate captain to investigate the theft of treasure from the shipwreck that stranded him there several hundred years ago. But just when she thinks she is closing in on the culprit, a murder takes place, and Claire and her friends quickly become the chief suspects. Can they recover the treasure, solve the murder and clear their names before all is lost?