The Robber Ghost (Cecilia & Michael #2)

the long version

Michael and Cecelia, friends and schoolmates, were as excited as everyone else in the Swedish town of Nordvick because of the mysterious theft of a large sum of money from the post office, located in the wing of the old castle. Of more immediate concern to them, however, was the weird, luminous figure they saw in the dark castle hall late one night. Surely it must be the ghost that, as everyone knew, had haunted the castle since the seventeenth century! But what was Bertil Dal, the unbearably arrogant new boy at school, doing outside the castle immediately thereafter?