"The Double Cross System in the War of 1939-1945", by J.C. Masterman.

I picked this up by pure serendipity-- it was mis-filed at my local used bookstore, and was the old Avon paperback-- cost $3.00.

I knew about the XX system and how the Brit secret service had turned every German agent in Britain during the war, but I didn't know the details. I had heard of Masterman in passing from other WWII historical references.

The real interest of this short book to me was in the erudition of the author and the pithy prose that was the result. So characteristic of educated British of the era, and so hard to find now. Masterman was a "typical" English polymath of the time, writer of mysteries, Oxford academic, world-class cricketer-- and head of the "Twenty Committee", that ran the double-cross system throughout the War.