Happy Hollisters and the Mystery of the Midnight Trolls

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. The letter said:
Dear Children,
Gram is writing to you in code. It is braille. How would you like to visit me and meet a special friend? Also, I wish you could solve the mystery of the midnight trolls.
With love, Gram
The children begged to go visit Gram and Gramp. But Mr. Hollister told them that he would be flying his new sailplane invention in a New York State competition in a few days and, if possible, in the big international sailplane meet in Iceland.
"We'll have to wait until Daddy tests his sailplane," Pete said.
Then Pam added, "While Daddy's in Iceland, we can go to Canada and solve the mystery!"
So in a few days, off they go to Canada where they meet a new blind friend who helps in their detective work. But when Mr. Hollister's sailplane invention is pirated, they all go on to Iceland to solve another mystery that turns out to be strangely linked to the first!