Dial-A-Ghost by Eva Ibbotson

The Dial-a-Ghost Agency finds good homes for ghosts. And Fulton and Frieda Snodde-Brittle are looking for a few frightening ghosts to "accidentally" scare their young cousin and heir, Oliver, to death. The ladies at the Dial-a-Ghost Agency have the perfect match: the Shriekers, two bloodstained and bickering horrors. But thanks to a mix-up at the agency, the Wilkinsons, a kind family of ghosts, arrive instead. Can they put a stop to the Snodde-Brittles' schemes before it's too late?
 
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