The Riddle of the Sands (1903) by Erskine Childers (http://en.wikipedia.org/wiki/Robert_Erskine_Childers) is an early spy/adventure novel set during the long suspicious years leading up to the First World War.
In spite of good prospects in the Foreign Office, sardonic civil servant, Carruthers, is finding it hard to endure the emptiness and boredom of his life in London. He accepts an invitation to join a friend on a sailing holiday in the Baltic, where they discover a German plot to invade England.
The story builds in excitement from there. It is the authentic detail and very fine characterization that make this such a gripping read. Highly influential on the whole subsequent spy genre. 288 pages in the Penguin Popular Classics paperback edition.
ISBN 978-0141197999
Erskine Childers