Counterspy by Matthew Dunn. Subtitled: A Spycatcher Novella, so I understand it's part of a series. Written by a former MI6 officer, so if nothing else it must be somewhat authentic. So far so good, in any case. It is fairly entertaining and a pretty easy read. I bought it a couple of weeks ago in a small charity second-hand bookshop. When I end up in a bookshop, however small, I ought to buy at least one book. I play it by ear and just buy something on a whim. So I bought this one, which cost me 50p. In my price range and, when it is so cheap, it is very unlikely to disappoint.