I agree with many of the other posters here. Cornwell has attacked the reputation of Sickert in this book, providing circumstantial evidence that in no way proves Sickert was indeed Jack the Ripper.
She interchanges "Sickert" for "Jack the Ripper" in a weak attempt at trying to fool the reader to believe the two were indeed the same person. She consistently uses "proof" such as "there is no way to prove otherwise" when accusing Sickert of being in the vicinity of the murders when they occur. She accuses that because he sketches women's body parts that he is fantasising in his sketches about dismembering their bodies. She says Sickert liked to take long walks, just as the Ripper supposedly did. She tries, rather weakly, to attribute every Ripper letter sent to the police, and even every letter written to the newspapers, to Sickert, simply because there is no proof that he is not responsible. And, Sickert used chalk, as the Ripper supposedly did after one of the murders. You could probably replace Sickert with at least 1,000 other individuals living at the time and come up with the same "proof."
Cornwell's writing style is very disjointed and jumps around from place to place and from time to time, which makes it difficult to put together timelines. Her tone is almost stream-of-consciousness, in that she seems to be discussing one aspect of the murders, then suddenly changes direction because she suddenly thinks of something else.
I'm sure the author put a lot of effort into gathering the information for this book, but perhaps she should stick to fiction and not accuse the dead and defenseless of one of the most infamous crimes in history.