William Le Queux 
German Spies in England 

Excerpt: "From the outbreak of war until to-day I have hesitated to write this book. But I now feel impelled to do so by a sense of duty. The truth must be told. The peril must be faced. Few men, I venture to think, have been more closely associated with, or know more of the astounding inner machinery of German espionage in this country, and in France, than myself. Though the personnel of the Confidential Department established at Whitehall to deal with these gentry have, during the past six years, come and gone, I have, I believe, been the one voluntary assistant who has remained to watch and note, both here and in Belgium-where the German headquarters were established-the birth and rapid growth of this ever-spreading canker-worm in the nation’s heart."