Cornelia Funke 
Ghost Knight 

Eleven-year-old Jon Whitcroft never expected to enjoy boarding school. He never expected to be confronted by a pack of vengeful ghosts either. And then he meets Ella, a quirky new friend with a taste for adventure…Together, Jon and Ella must work to uncover the secrets of a centuries-old murder, while being haunted by ghosts intent on revenge. So when Jon summons the ghost of the late knight Longspee for his protection, there’s just one question – can Longspee really be trusted? A thrilling tale of bravery, friendship – and ghosts!