In the broadest sense, there are two kinds of heroes: the man of action and the man of thought. There's necessarily degrees of overlap in every main character - the man of pure action is a lunkhead, the man of pure thought a wimp - but if I name a popular fictional character, most people can accurately discern which quality is predominant. Batman, for example, is in peak physical condition and an expert martial artist, but he's a man of thought. It's why he's become a memetic badass over the last decade. Superman calls him the most dangerous man on earth because of the power of Batman's mind. Anyone can learn how to fight, but only a gifted mind could become the world's greatest detective. Batman's intellect is what puts him on equal footing with super powered villains.
Sherlock Holmes is the essential man of thought. Sir Arthur Conan Doyle was bound by the strictures of 19th century fiction, not to mention the fact that he quickly dashed off the stories to make money and stave off boredom in the early days of his medical practice. That means the original Conan Doyle stories are seldom meticulous case studies. They contain more fisticuffs and gun fighting than you might expect if you've never read them. In the popular imagination, Holmes, wearing a deer stalker cap he never wore in the stories, peers at the ground with a magnifying glass for a few minutes before pronouncing that the killer is that man over there because the biscuit crumbs on his lapels and uneven tan lines on his hands prove that his alibi is entirely false. We're fascinated by Holmes's Aspergery nature.
Every great hero needs a great villain. Professor James Moriarty was invented by Conan Doyle as a way to kill off Holmes, with whom he'd grown tired of writing. Moriarty is the Bizarro Holmes, a man of supreme intellect who devotes himself to crime instead of justice. He only appears once in the Holmes canon, but he has become Holmes's arch nemesis. Moriarty, by Anthony Horowitz is a Holmes novel without Holmes and Watson. The story is narrated by Frederick Chase, a New York Pinkerton detective who is in pursuit of the American master criminal Clarence Devereaux. His investigation leads him to the Reichenbach Falls and a meeting with Scotland Yard Inspector Athelney Jones. Devereaux was apparently trying to arrange a meeting with Moriarty for the purposes of joining forces into one gigantic trans-Atlantic criminal empire. Moriarty's demise at the hands of Sherlock Holmes prevented that meeting, but so far only the police know of his death. Devereaux and Moriarty have never seen each other before, so Chase and Jones concoct a plan to impersonate Moriarty and pursue Devereaux, to bring down his burgeoning criminal enterprises in London.
My personal test for a good detective story is how easily I can figure it out before the end. A good detective writer should always ensure that the necessary clues appear in the text so that, even if the reader can't remember them all upon the first reading, a second reading will confirm that they were there all along. This story pulls it off in spades. Indeed, I lol'ed at myself for not figuring it out sooner. The twist has been done before but it was a great pleasure to see it done so well with characters I know and love. 4/5, would read again.