Huntingtower by John Buchan

In HUNTINGTOWER there is a retired Glasgow grocer who, starting out for a walking tour with a knapsack and a poetry book, finds himself, within twenty-four hours, up to the neck in one of the wildest plots of adventure it ever entered the heart of man to conceive. There is a beautiful Russian princess abducted by Bolsheviks and immured in the lonely castle by the sea; there are hidden jewels, a villainous innkeeper, with a gang of 'tinklers' keeping watch over the princess until the arch-villain…