Dial a Ghost by Ibbotson, Eva

‘Get me some ghosts,’ said Fulton Snodde-Brittle. ‘Frightful and dangerous ghosts!’ Fulton has gone to the Dial A Ghost agency with an evil plan. He wants to hire some truly terrifying ghosts to scare his nephew Oliver to death. The Shriekers are the most violent and sickening sceptres the agency has, but a mix-up means the kind Wilkinson ghosts are sent in their place. Now Oliver has some spooky allies to help him outwit the wicked Snodde-Brittles… ‘You`ll love this chain-rattling, blood-oozingly hilarious story` Daily Telegraph