I would have thought that pastiches of the Sherlock Holmes oeuvre, of which this claims to be the first authorised by the Conan Doyle estate, would be one of the easiest to pull together. Each of the stories follows a fairly rigid pattern, and there are a series of boxes for any author to tick - Holmes with violin, tick, cocaine addiction referenced but not indulged, tick, London fog, rattling coach drives, cheeky Cockney urchins , Mrs Hudson making tea, Mycroft being inscrutable, and of course the dazzling deductions based on flimsy evidence (but never guesses, oh no). And with Dr Watson you have the most affable and gentle of narrators, always comfortably behind the pace, leaving the reader a sense of superiority - we can work out which is the Holmes in disguise character before he does, how the locked room is escaped from, etc. Horowitz sinks into the comfort of these clichés with an almost audible sigh, and the reader is granted 400 pages of predictable, unchallenging nonsense.
Another Conan Doyle tradition that Horowitz follows religiously is the tendency to not bother too much with plausibility. Raymond Chandler in the Simple Art of Murder famously described the Holmes stories as "mostly an attitude and a few dozen lines of unforgettable dialogue" and elsewhere, although I can't find it now after at least five minutes on Google, picks apart the plot of The Hound of the Baskervilles, pointing out just how many absurdities and improbabilities the plot contains. Horowitz maintains this tradition with a plot that depends utterly on people being in the right place at the right time - for example Holmes's escape from prison depends on him bumping into a prison doctor who he knows from a previous case, not to mention the bizarre behaviour of the Irish gangster who marries a man to exact revenge upon him, which as a plan has a number of flaws in it (ie a dependence on him being gay and thus not want sex with you, but prepared to marry you nonetheless!)
There is some updated knowingness here - we meet Moriarty, but he doesn't play a part in the plot - and the concern for the underclass (for example the street children Holmes uses as his eyes and ears in back streets) missing in the Conan Doyle. Watson's narration is set many years in the future, after Holmes's death, which as a device adds nothing to the novel. As a deviation from the Conan Doyle tradition of having a near contemporaneous narration this seems a strange one to choose.
Nevertheless, Horowitz does a competent job throughout, without at any stage dazzling or impressing - the nearest he comes is during the unveiling of the villain at the close, which as described above doesn't stand up to much if any scrutiny. In the back of my mind throughout the read was the way the recent TV series had grasped all of these conventions but not proven shackled by them, managing to remain true to the spirit of the original but reinventing Holmes for a modern generation. The House of Silk suffers significantly by comparison