How can I rate this? It’s too hard! As far as the writing is concerned, and the recreation of Arthur Conan Doyle’s voice: 5/5. The cleverness of the plot: 5/5. But the actual twist in the plot?: 0/5. I did not like it at all.
So, my recommendation is this: if you adore the character of Sherlock Holmes, and do not want his image sullied in your mind, do not read this book. If, however, you are open to different interpretations, be my guest!