A Divided Spy by Charles Cumming St. Martin’s Press, $26.99, 356 pages, ISBN 978-1-250-02104-5 For some arcane reason . . . known only to themselves and the cosmos . . . the Brits are the masters of the spy novel universe. One only has to think of the names Ian Fleming and John LeCarré, or their iconic fictional spies, James Bond and George Smiley to know what we mean.