I'm a little afraid to talk about the main message it sends (it's very sensitive and I don't want to upset anyone) so I won't focus on that. You'll see what I mean if you read the book.
The one major thing that's sticking out to me on this reread is how dark the story is. I remember when I first read it and the convoluted mystery confused me a bit, but this second reading is really showing me where all the little pieces of the puzzle fit together. And that's fascinating to me!
And the subject matter is something I have not seen any author of any book touch upon. I believe that it's sensitive enough so that no one dared to. Yet Horowitz did, and did so masterfully. Why is this the case? How are some authors able to take such huge risks in writing yet others cannot? Sometimes the risk doesn't pay off.
I remember after I read Divergent (which I hated with a passion) and the final book came out, and people were furious because the author had taken a certain risk - a risk that's practically unheard of, unless done correctly. My guess (I haven't read the book myself) is that she did not.
I read another book under Project: Sherlock that took the biggest risk I've seen any of the pastiches take.... but it failed, because the author was using a character that wasn't his and he damaged that character.
So what? What does that mean?
My point is, it's okay to take certain risks in writing, but for the risk to pay off you have to know what you're doing. You also need to think of your audience when writing, at least put them in the back of your mind. I know it's her book, but the author of the final book of the Divergent trilogy was not thinking about how her audience would react to what she did. I mean, people were so upset that they swore off her books and sent her death threats....
I'm sure Horowitz knew what he was doing when he wrote The House of Silk. I think what he did in the preface of the book was a way of protecting himself, because the narrator lampshades how the events in the book were of an extremely sensitive nature. I think Horowitz was saying to his audience, "This mystery will shock you and perhaps upset you, but I'm writing this book the way I feel it needs to be told. So get ready."
The ultimate thing here is that taking risks in writing is something that a lot of authors do. Whether they do it right or not depends on the book they are writing and how they approach the subject. But I believe that Horowitz rose to the occasion and did it right.
This guy did it right and his book is a masterpiece as a result. I recommend The House of Silk to anyone looking for a great mystery.