Let's Boo This! by Sibéal Pounder

The night Joe Bones spots a cloaked figure leaving his gran's house, he's sure that something is afoot. So, when he hears that his gran has passed away he's convinced the mysterious man has something to do with her death. Then Joe discovers that his gran was keeping a secret - she was part of an ancient society called Fright Club, investigators of crimes in supernatural hidden worlds. So, Joe convinces his three best friends, Hal, Debbie and Rose to revive the club and track down the mysterious figure to find out what really happened to his gran.