Wild chamber - Christopher Fowler

Our story begins at the end of an investigation, as the members of London's Peculiar Crimes Unit race to catch a killer near London Bridge Station in the rain, not realising that they're about to cause a bizarre accident just yards away from the crime scene. And it will have repercussions for them all
One year later, in an exclusive London crescent, a woman walks her dog - but she's being watched. When she's found dead, the Peculiar Crimes Unit is called in to investigate. Why? Because the method of death is odd, the gardens are locked, the killer had no way in - or out - and the dog has disappeared.
So a typical case for Bryant & May. But the hows and whys of the murder are not the only mysteries surrounding the dead woman - there's a missing husband and a lost nanny to puzzle over too. And it seems very like that the killer is preparing to strike again.
As Arthur Bryant delves in to the history of London's 'wild chambers' - its extraordinary parks and gardens, John May and the rest of the team seem to have caused a national scandal. If no-one is safe then all of London's open spaces must be closed
With the PCU placed under house arrest, only Arthur Bryant remains at liberty - but can a hallucinating old codger catch the criminal and save the unit before it's too late?