This is a superb book if you like esoteric detail - about boats, channels, sandbanks, marshes, and the political situation before WWI. I loved it, and I would also recommend the very worthy film adaptation with michael York.
It is fascinating that Childers wrote this as a patriotic attempt to warn the British establishment about Germany's warlike intent - yet he was later executed as a traitor because of his involvement in the Irish struggle for independence. What a rich irony.
But read the book - it is a superb period piece and a well-paced thriller.