When people talk about early detective novels, and Dickens's influence on the genre, they usually tend to bring up Inspector Bucket of Bleak House. The Inspector has been called "the first important detective in English literature," and there's little doubt he merits the description.
But there's at least one other detective figure in Dickens's work who seems to be almost completely overlooked: Sissy Jupe in Hard Times.
I realize, of course, that Sissy is not a professional detective. But the detective canon is full of amateur sleuths, and I would argue that the humble "stroller's child" deserves to be listed among them.
(I'm going to put some major spoilers below the cut, so proceed at your own risk!)