Cornwell - a crime reporter, author and former medical examiner - has done her research in terms of the paltry pieces of evidence left after 120 years. She examines the photos and letters, attempts DNA tests on envelopes and stamps and chases down clues that were undiscovered during the time of the crimes.
Her conclusion (no spoiler here) is that Walter Sickert, a prominent English artist of the time, did the Ripper murders. I'd be more convinced if she'd come to the conclusion at the end of her research, instead of spending the narrative making conjecture that fits evidence to her suspect.
The book is well-written and contains interesting medical, sociological and psychological forensic information, however the information is used to give a questionable conclusion. So, read it for the details and nevermind the conclusions.