Paul Mason 
The Twins, the Ghost and the Castle [PDF ebook] 

Two abandoned children make their home in a castle – which turns out to be haunted by the Duke of Wellington! The ghost is a particularly determined spook who helps them hide from the castle”s caretaker. But when developers want to turn their home into a spa, everyone who loves the castle must band together to save the day. Comic ghostly fun for newly confident readers aged 8+ and those looking for a fast, fun read.