Ice Ghosts Mystery

I first read this book when I was 12 years old and I've read it innumerable times since. It's impossible to put down and more gripping than most mysteries written for adults. Mrs. Byrd and her three children, Oriole, Peregrine, and Mab have just received a mystifiying telegram from the now-missing Professor Byrd. He's disappeared in a remote valley high in the Alps. In his telegram he mentions "Tce ghosts." Does he mean Ice ghosts or The ghosts? Mrs. Byrd hurriedly packs her family onto an airplane, and they're off to investigate the mysterious disappearance and the meaning of the telegram. With the help of a local boy, a bumbling folklorist, and an instructor from the nearby climbers' school, the Byrds must solve both mysteries if they ever want to see their father again.