Panic — Helen McCloy

When Uncle Felix dies of a suspected overdose of digitalis, his niece Alison accepts the offer of a remote mountain lodge for the summer to get away from the tragedy.

But there are strange noises in the night and sinister visitors - and she discovers that the previous tenant was driven insane. What also transpires is that Uncle Felix had devised what he claimed to be an unbreakable cypher. The Pentagon is interested in this claim, and Alison has a fragment of a clue found beside her uncle's bed. In the mountains she wrestles with the puzzle. But solving it will put her life in grave danger ...