Sweeney Todd: The Real Story of the Demon Barber of Fleet Street, by Peter Haining
I first read this one several years ago, way before the questionable film adaptation, when the musical was more popular among theatre nerds than mainstream Glee fans. This would be one of the better books investigating a killer that I've had the fortune to read. What's more, it theorizes that Sweeney Todd was a real person, and pretty much convinced me when I read it.
Jack the Ripper has gotten pretty passe, and though H. H. Holmes is a fascinating individual, neither of them (from what I have gathered) served their victims for breakfast or lunch. I don't mean served to them, I mean literally served them up in meat pies.
This book was also an excellent source for a college essay I wrote a few years ago, thoroughly convincing my professor I was a disturbing individual who should best be avoided. It's a 'nonfiction' work that manages to be incredibly entertaining in the way you'd expect a suspense novel to be. By the end of it, I actually believed Sweeney Todd may have been a real person. But I was also 10 the first time I read it, so I'm not really sure you can judge from that opinion.