Clue in the Castle Tower: A Samantha Mystery (American Girl Mysteries Series)

When Samantha and Nellie travel to England with Grandmary and the Admiral, they visit an old friend of the Admiral's--an English knight who lives in an ancient manor house, next to castle ruins that are rumored to be haunted. They glimpse strange sights in the castle tower at night--and learn that valuable old books are disappearing from the manor's library. Samantha and Nellie join forces with the knight's mischievous twin nephews to find out who--or what--is behind the strange goings-on.