The greatest detective in fiction against the most feared murderer in modern history: a mouth-watering prospect.
This short novel, taken directly "from the notes of Dr. Watson" (a nice way to explain the differences in style when compared to Arthur Conan Doyle's canon), is well written up to some three-quarters into it, when it becomes slightly over-the-top, although this is explained by Dr. Watson's nervousness.
The revelation of Jack the Ripper being Professor Moriarty is quite predictable, but the fact that both such fiends are no one else than Holmes himself is quite shocking - although the author builds up nicely towards it.
Dibdin also managed to explain why Doyle's stories set after Holmes's "return from death" are clumsier than those set before that event: they were a work of fiction (even in the fictional world inhabited by Holmes and Dr. Watson), because Holmes did indeed die at Reichenbach falls, alongside Moriarty. Who is Holmes himself anyway. Kinda mind-boggling, really.