Awful Auntie by David Walliams. Ages 7+.
Awful Auntie (2014) opens with a mystery: Stella Saxby, a young aristocratic girl, wakes up in her family's crumbling manor house with no memory of what happened to her. Her parents are gone. Her enormous, horrible Aunt Alberta has taken charge — along with her pet barn owl, Wagner.
Stella gradually recovers her memory and discovers the truth: her Aunt Alberta caused the car crash that killed her parents, in order to inherit the manor house. Stella herself was left for dead. Stella teams up with Sab, the ghost of a cockney chimney sweep boy who died in the house long ago, to outwit her aunt and reclaim her home.
The novel combines mystery, comedy and warmth. Aunt Alberta is one of Walliams's most physically imposing villains — large, loud and entirely without conscience. Sab provides comic relief and unexpected depth.