(From the publisher):
The Happy Hollister children are off on another exciting mystery! This time it all starts with a letter written in a funny code from Grandmother Hollister who lives in Canada. While their father goes to Iceland to compete in the big international sailplane meet, the children decide to visit Gram and Gramp in Canada to help solve the mystery of the midnight trolls.
Original title: The Happy Hollisters and the Mystery of the Midnight Trolls
Genre: Fiction→ Children & Young Adult→ Mystery