The Valley of Fear is the last Sherlock Holmes novel, in which Holmes and his assistant Watson finally begin to understand the threat their arch-nemesis Moriarty presents.
The Penguin English Library Edition
The deadly hand of Professor Moriarty once more reaches out to commit a vile and ingenious crime, but a mole in Moriarty's criminal organization alerts Sherlock Holmes of the evil deed by means of a cipher . . .
When Holmes and Watson arrive at a Sussex manor house they appear to be too late. The discovery of a body suggests that Moriarty's henchmen have been at their work. But there is much more to this tale of murder than at first meets the eye.