Dial a Ghost by Eva Ibbotson

"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.

But Oliver likes ghosts. And the peculiar ones he meets at Helton Hall decide at once to help him outwith the wicked Snodde-Brittles . . .