Scout: The Treasure of Rodensteyn Castle by Piet Prins
When they are invited to spend a few weeks with Uncle Arnie at the seashore, little do Tom, Carl, and Bert realize that they will be staying in the hunting lodge of a medieval castle. Uncle Arnie is quite a storyteller. Soon the boys find their imaginations and the lodge peopled with ghosts from Rodensteyn Castle, a castle long ago buried under the sand. But do ghosts leave tracks, tracks that Scout follows into the sea? Are the legends of the ghost of Sir Isobald and the treasure of Rodensteyn Castle true? Is it a ghost that Tom spots in the dunes at night? Can Scout's nose, no matter how keen, help the boys solve a mystery over 400 years old? Past and present, fact and fiction merge as Tom and his friends search for a treasure that has outlasted the ages.