Somewhere around a thousand authors must have tried their hands at Sherlock Holmes, as he's been so long out of copyright, so I don't really understand how the Arthur Conan Doyle Estate really feels they have any particular right to choose an author to write an "authorized" Sherlock Holmes novel, but I must say they did a pretty good job in choosing Horowitz.
It's a long time since I have actually read Conan Doyle's novels, but this novel certainly reads the way I recall those stories. Holmes, Watson, Lestrade, Mrs. Hudson and even Moriarty, seem totally authentic. The two intertwined cases show Holmes usual brand of brilliance in detection - though Horowitz' version relies on a whopping great coincidence to tie them together. I can't help thinking Conan Doyle would not have approved.
Only once in a while did a glaring anachronism manage to creep in - I wonder if it's actually easier for an American to write about 1890 London than it would have been for someone living in London today. The one that jumped out at me was a police officer giving the arrested man his rights: "you are not obliged to say anything unless you desire to do so, but whatever you do say I shall take down in writing..." This wouldn't have happened prior to the Judges Rules of 1912, at least.