Of all the Holmes pastiches I have read (and there have been many), Horowitz has, I believe, achieved the most authentic Watsonian voice. For most of the time, it is possible to believe the book was written by Conan Doyle, the master storyteller, himself. All the regular characters are there - Inspector Lestrade, Mrs Hudson, brother Mycroft - and as a Holmes fanatic, I wasn't conscious of any of those jarring inconsistencies that mar many a Holmes tribute. The plot is complex and well written, and we see Holmes both as the calculating thinker and as the man of action. The Holmes/Watson relationship is very faithfully portrayed.
However, I felt that sometimes Horowitz allowed the tone to stray quite far from the originals. For example, Watson's concern for the contrast of rich and poor, his reflections on the street urchins, smacked more of Dickens than Conan Doyle. Suddenly the Baker Street Irregulars are no longer the cheeky, street-smart gang of old; now they are to be pitied for their poverty and the harshness of their lives. All true, of course, but not in keeping with the originals. I also felt that the main strand of the plot was well outside the bounds that Conan Doyle would have set and as a result in the latter stages it got more difficult to forget that this was not the genuine article.
In the Kindle version, there is included a very interesting essay by Horowitz where he describes how he came to write the book and lays out the ten rules he set himself, before beginning to write, to try to ensure an authentic flavour. He admits that he broke one or two of the rules along the way and I feel that was a pity - had he managed to stay within them I believe the end result would have been as close to perfect as any homage could be.
Notwithstanding these criticisms, which I am sure would only bother other Holmes pedants like myself, I think this is a very good read, well written, well plotted and full of interest. The best faux-Holmes I have read, I would recommend this to existing fans and newcomers alike.