"The Cuckoo's Calling" (Mulholland Books), by Robert Galbraith (J.K. Rowling)
Full disclosure: I never would have guessed.
I've read every book J.K. Rowling has published, some of them several times over. OK, all of them several times over. And still, I doubt I would have picked up on anything that would have made me think that SHE was the author behind "The Cuckoo's Calling."
I almost wish I hadn't known, so I wouldn't have read it with some of my attention in search of clues that would have made me suspicious that the book's author wasn't, in fact, an ex-military police officer and was actually one of the world's most famous storytellers.