There are 4 books in the The Cthulhu Casebooks series. The newest book is Sherlock Holmes and the Highgate Horrors which came out in 2023.

1
Sherlock Holmes and the Shadwell Shadows

Book cover of Sherlock Holmes and the Shadwell Shadows
It is the autumn of 1880, and Dr John Watson has just returned from Afghanistan. Badly injured and desperate to forget a nightmarish expedition that left him doubting his sanity, Watson is close to destitution when he meets the extraordinary Sherlock Holmes, who is investigating a series of deaths in…
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2
Sherlock Holmes and the Miskatonic Monstrosities

Book cover of Sherlock Holmes and the Miskatonic Monstrosities
His health nearly broken after a decade combating eldritch entities and evil cultists, Holmes must marshal his inner resources in order to tackle his most dangerous and terrifying case yet. Miskatonic University in Arkham, Massachusetts, has a new resident academic, a certain Professor James Moriarty. In tandem with a young…

4
Sherlock Holmes and the Highgate Horrors

Book cover of Sherlock Holmes and the Highgate Horrors
The stunning new novel from the New York Times bestselling author of The Age of Odin, in which the worlds of Arthur Conan Doyle and H.P. Lovecraft collide.