Sherlock Holmes and the Sussex Sea-Devils
It is the third and final book in "The Cthulhu Casebooks" trilogy following "Sherlock Holmes and the Shadwell Shadows" and "Sherlock Holmes and the Miskatonic Monstrosities" . 
<triplet> Sherlock Holmes and the Shadwell Shadows <subj> Sherlock Holmes and the Miskatonic Monstrosities <obj> followed by