Powers of Darkness by Fred M. White

Clench's visit to Somerset House had been something in the nature of an afterthought. In his own words he had cast a fly over a stream with the waters of which he was not very familiar. There was a possibility that he might land a fish, but he had not expected to hook a fine salmon.

There was a chance of discovering something useful between the lines of Martin Faber's will, and his expectations had been exceeded.