I don't know why some of the other reviewers here can't accept Cornwell's theory on Sickert. To me it makes perfect sense. I hope that she will eventually find something to prove it, because that's what is missing to date.
Through Cornwells narration we can vividly relive the fear that reigned at the time. She uses all her skills and modern crime investigation techniques to present the case. She also gives historical insights of the times of the murders. Moreover, the book contains plenty of photographs (of Sickert's works as a painter, of the vicitms (!), of Sickert himself, of the famous Ripper letters sent to the London police and of other facts related to the crimes).
The last part, however, seems to have been put together in a rush. Everything is superficial in the end. Cornwell no longer gives her own opinion, she only quotes others. She probably had to meet a deadline, which is a pity as the book started out just great.