The House of Silk is over hyped which probably caused me to rate it lower than I might have had it not been touted as the second coming of Conan Doyle.
First, there have been many authorized additions to the Canon. This is hardly the first and most definitely not the best. My personal favorite is The Exploits of Sherlock Holmes by Adrian Conan Doyle ( son and 'literary executor' of Arthur Conan Doyle ) and John Dickson Carr ( well worth reading for his own stories under his name and his pen name of Carter Dickson ).
Second, Horowitz seems to have attempted to modernize the attitudes, actions and style to suit today's audience. We lose some of the Victorian charm and the feeling of being in another era. I find that detracts from the book.
Third, both the Holmes' seem somewhat out of character. Sherlock is less the analytic machine, cold and emotionless and more moody, brooding and a bit weepy. It takes too long for him to return to character. Mycroft is more concerned with keeping his position than doing what is right for the country. It feels like Horowitz takes book time to allow his characters to find their pace.
Fourth, Horowitz finds it necessary to provide a back story in the preface that is not necessary to the story. I didn't find that it added much beyond page count.
In spite of all those negative words, Horowitz is an experience and capable writer. It may take a while to for the reader to discover that. And, eventually there is a sufficiency of action and mystery to keep the puzzle solver distracted and attracted.
The bottom line is that it is a book worth reading. It's just not as good or as unique as the publisher would like us to believe.