This book is the basis for the Alan Moore comic (and thus - loosely - the lamentable Johnny Depp vehicle) From Hell, and as an appreciator of the comic, I figured I would never have an reason to bother reading the original history it was based on. Then I ran into it one Saturday in a used bookstore, and what can I say, I'm a sucker for old paperbooks.
Stephen Knight gives a journalistic account of the Ripper murders, which is to say he acquaints you with a lot of the facts of the case, while also making a sustained argument for his particular (and particularly outlandish) interpretation of their meaning. I'm given to understand this is widely discredited now as being a product of his (and his main informant's) paranoias about the Freemasons, albeit bringing to light some formerly overlooked facts of the case in the process. Regardless of all these details about being a "plausible" or "historical" account, Knight tells a damn good story, and for my money the most entertaining explanation of the Ripper murders you're likely to find. If you're bothered by the thought that it's hard to see the truth for Knight's theory, then maybe you shouldn't be reading a book about Jack the Ripper. Knight rightly points out that a century removed from the crimes we're no closer to the truth mostly because of authors overlooking facts because of their own attachment to theories (which Knight points out, mind you, before giving his own biased version of the facts). I long ago settled my unease about this by reading Alan Moore's brilliant 2nd appendix to From Hell - an illustrated account of the Ripper historiography (no, seriously, it's awesome) - and thus I'm content to read a book like Knight's without worrying too much about whether it's true.