Children's Books, Contemporary 

London, Today: Arthur Drake is haunted. He is haunted by memories from 1936 that are not his own. He is haunted by a girl from school who turns up where and when he least expects it. He is haunted by the ghosts in a deserted and derelict old house... London, 1936: Whatever your problem, the Invisible Detective can find the answer. Only four children know the truth about this mysterious private investigator... because they created him. Now they solve crimes and mysteries in his name. Investigating a strange death and a haunted house, Art and his friends are drawn into a mysterious world where nothing is what it seems, and nobody can be trusted. There are monsters on the streets of London, dressed as soldiers and trained to kill. As a terrifying plan is put into action, only the Invisible Detective can stop the Ghost Soldiers... - from Amzon