One imagines that MI5 was busy during World War II. But not too busy, it would seem, to take the time to investigate Agatha Christie. Why? Well, says the Guardian,

The answer, it can now be revealed, lay in the name of a character in her wartime novel N or M, whom she called Major Bletchley. He appears in the book as a friend of Christie’s pair of detectives, Tommy and Tuppence. In the book, published in 1941, N and M are the initials given to two of Hitler’s agents as Tommy and Tuppence hunt for the enemy within. Major Bletchley comes across as a tedious former Indian army officer who claims to know the secrets of Britain’s wartime efforts. Christie happened to be a close friend of Dilly Knox, one of the leading codebreakers at Bletchley Park. MI5 was concerned that the major’s inside knowledge of the progress of the war was based on what the codebreakers knew about Hitler’s plans. Had Christie mischievously named the character Bletchley because Knox told her what was going on there?

The codebreakers at Bletchley Park in Buckinghamshire had broken German Enigma machine cyphers, enabling Churchill and his military commanders to know what the enemy was planning. Berlin believed Enigma was unbreakable, making it all the more essential to ensure that only a very small circle of people knew what the codebreakers at Bletchley were up to.

What worried MI5 even more was that it was Knox who had just broken the Enigma machine cypher used by German secret service officers sending spies to Britain.

It is almost unthinkable to imagine equal concern being lavished on the work of a modern bestseller; James Patterson and John Grisham somehow don’t seem likely tools of espionage, although it’s tempting to imagine government agents poring over the bestseller list in search of security breaches. In any event, MI5’s fears were unfounded. When confronted, Christie responded, “Bletchley? My dear, I was stuck there on my way by train from Oxford to London and took revenge by giving the name to one of my least lovable characters.”