In Robertson’s fifth series novel, The Baker Street Jurors, a British solicitor whose chambers are at 221B Baker Street, London, is legally required to respond to letters addressed to Sherlock Holmes.

Where did the series come from?

I was taking a screenwriting course, way back in 1980, and desperately scouring newspapers for an idea. I came across an article about a British bank that decided in 1932 to build its huge new headquarters on Baker Street, which had just then been extended to add a new block—the 200 block. You can guess what happened next. 221B Baker Street—an address that did not exist when Conan Doyle created Sherlock Holmes—now had a real limestone-and-plaster presence. And letters that the Royal Mail had for years delivered either to Scotland Yard or to Conan Doyle himself now had a real address on Baker Street to go to. This was too good a premise to pass up.

How do you account for people still sending letters?

You might think that by the 1930s, the letter writers would have begun to suspect that if the great detective did exist, he would be getting on a bit in years. And in subsequent decades, it should have been completely clear that whatever corporeal existence he might have once possessed was long since gone. But not so. For the purpose of my novels, there are four kinds of people who still write to Sherlock Holmes as though he is real—the very young, the very old, people who for one reason or another have a slightly distorted view of reality in some particular area, and people for whom Sherlock Holmes is their first introduction to English-speaking culture.

You threw out your first draft of the series’s first book?

I actually threw out one complete draft, and also several earlier partial ones where I had attempted to write in the style of Sir Arthur Conan Doyle. I could not—and also did not really want or need to, given that my books are not actual pastiches. My novels take place in a universe where the letters are real, but Sherlock Holmes is not.

The Princess Bride and The Thin Man are two of your favorite books—in what way did they influence this series?

I loved The Princess Bride because of how William Goldman used all the icons and traditions of a particular genre and made them hilarious without making them any less sincere and genuine. I’m a fan of Dashiell Hammett generally, but The Thin Man in particular, because of the interplay between Nick and Nora Charles. I strive for that with Reggie and Laura in my first four novels, and with Nigel and Lucy in The Baker Street Jurors.