“Lethal White,” the fourth volume in Robert Galbraith’s hugely popular Cormoran Strike mystery series, goes on sale Sept. 18. But who, exactly, is Robert Galbraith? The British author has never appeared at a bookstore event; you won’t find his photo on the books’ jackets. What little we know about Mr. Galbraith comes from his good friend J.K. Rowling, who has talked about him on Twitter — and from his own website, which details his military background. But until now, the reclusive author has been shy about granting interviews.

Can you talk about your previous career? I understand you may have had some sort of top-secret clearance. You may not be able to be specific, but even a broad-brush portrait would be helpful.

I previously worked in the plainclothes S.I.B. (Special Investigation Branch) of the Military Police. After leaving the army and while writing my first novel, I worked in the world of private security, which explained my initial reluctance to be photographed, interviewed or take meetings, lest my uncanny resemblance to J.K. Rowling be noticed.

An ex-military friend of mine was ready to go to my publishers to shake hands as Robert, because most people at Little, Brown didn’t know who I really was. However, Robert was unmasked before that happened, somewhat to my friend’s relief.