Wild Robert, by Diana Wynne Jones, 1989

Heather hates living at Castlemaine, where her parents are curators, and where tourists flock like sheep, leaving her no private place except the mound at the edge of the lawn. When Heather accidentally summons from the mound the glowing form of Wild Robert, once the heir to Castlemaine but for some treachery on account of his witchcraft, she finds she has far greater problems than tourists on her hands. A brush with the past that offers as much complication as romance, short and tightly written early middle grade fantasy. Strongly recommended.