The Mystery of Cabin Island by Franklin W. Dixon

In spite of Joe Hardy's predictions that the marauder would be back for his notebook, that afternoon and the next day passed uneventfully on Cabin Island. No one had appeared in the vicinity of the rocks, for the boys examined the place carefully in search of footprints and the snow was still unbroken.

The mystery surrounding Hanleigh, John Sparewell, and the Bender postage stamp collection was gradually receding into the background. But to the Hardy boys it still remained a matter of great concern, especially to Frank. Each evening he sat down and puzzled over the strange cipher, vainly trying to solve the mystery it presented.