It’s no secret that Richard Castle isn’t a real person.He is just a character in a popular TV show, ABC’s Castle, and he is played with panache by actor Nathan Fillion.But that hasn’t prevented Castle the character — a mystery writer who solves crimes alongside his muse, NYPD Detective Kate Beckett (Stana Katic) — from publishing a very real bestselling series of novels.Dismiss Castle’s Nikki Heat thrillers as being a mere promotional tie-in if you will. Yet it’s hard to make light of the fact that, for the past four years, the action-packed “Heat” novels have topped the New York Times and USA Today bestseller lists, moving more than 2.3 million copies.Book three in the series, 2011’s Heat Rises, debuted at No. 1 on the New York Times list.Book five, Deadly Heat (Hyperion, $26.99), hits store shelves Tuesday.What’s unique about these books is that they’re not “novelizations” of episodes. Nor are they even about Castle and Beckett. They’re the books that Castle writes in the show.It’s fun for Castle fans to see what the guy has been writing all this time.And it’s fun for fans to imagine Kate Beckett’s slightly peeved reaction: at how she has been transformed onto the page into the sexy sleuth Nikki Heat; at how she and Castle (as dashing Jameson Rook) were depicted as romantic partners from the start.It’s also fun to check out the packaging, with Fillion pictured on the back cover as the author and a bogus Richard Castle bio inside, and to note how committed everyone is to playing this elaborate game.It should come as no surprise that, when we asked for an interview with Richard Castle, we were told he’s unavailable. So instead we chatted with Andrew W. Marlowe, creator and executive producer of Castle, about the unlikely success of the Nikki Heat books. What sparked the idea of publicizing the Richard Castle books in this way? Is there a literary precedent for what you’re doing?I’m not sure there is a precedent for what we’re doing — though if memory serves, I think Jessica Fletcher of Murder, She Wrote also published a couple of books.When I was pitching the show, the idea of Richard Castle writing novels was in the back of my mind. It seemed like a natural extension to the storytelling that we were presenting in the show. After all, since the show’s premise is that Castle teams up with Beckett because he’s looking for inspiration for his new book series, we felt the audience would enjoy seeing the fruits of his labor.We’ve tried to honor the premise, so for loyal fans who read the book series, they’ll be able to see the moments from various shows that Castle has used for inspiration — locations, characters, situations, not to mention relationship issue. Were you surprised by success of these books?We were all delighted when the books became New York Times bestsellers, Richard especially. We never really thought of the books as a ‘tie-in’ or as ‘merchandising,’ but as a way to extend the experience of the show into a new arena.And given Richard Castle’s stature as a writer in the world of the show, we were committed to making sure that the books reflected the level of quality that readers would find in any other mainstream bestselling mystery novel. Luckily, Richard Castle felt the same way. Do you happen to know if faithful viewers are the vast majority of people driving books sales? Or are the books finding readers unfamiliar with the show?We don’t have any data, but I’m sure show viewers comprised the majority of our early audience for the books. Although I have met folks on vacation reading the Nikki Heat books poolside who have never heard of Castle the TV show.We like to imagine that there’s synergy going on and that the folks who have discovered the novels independently have gone on to become viewers. Is it true that Nathan Fillion has done book signing appearances, albeit signing his own name, not Richard Castle’s, and never pretending to be anything other than the actor on the show?We’ve done some signings with Nathan, where he does sign his own name. His point of view is that Richard Castle is an imaginary character: If you want Castle’s signature, you can just as easily sign it yourself.Despite this bizarre insult and affront to his existence, Richard Castle still remains a Nathan Fillion fan. Is it also true that some people steadfastly believe that Nathan is actually the author?We have run across some folks who do believe that Nathan is the author of the books. This is probably due to Nathan’s striking similarity to Mr. Castle.