Dennis Wheatley wrote a tremendous number of thrillers, but this is perhaps his best. A story of his 'Modern Musketeers' the Duc de Richleau, Simon Aaron, Rex Van Ryn and Richard Eaton, it takes place in the Spain of the 30s. The Civil War is raging, with both sides prepared to go to any length to win.
The Duc de Richleau is asked to find an enormous stash of gold, hidden somewhere in the war torn country. Determined to beat the communist forces, he asks his friends to help him find it. However, he finds that two of his friends are sympathetic to the communists. Very soon the former comrades find themselves trying to outwit one another in a potentially lethal treasure hunt.
This is a real edge of the seat read. The action-packed plot twists and turns right until the end, but there is also a real depth of feeling (not always Wheatley's strong suit). On top of this, there is an enormous amount of background about the conflict, which is fascinating all by itself. If you are only going to read one Wheatley book, this should be it.