Looking at the notes of the Holmes books, I read up on a couple of the stories that had a "O" next to the title which I assume where clues or stories Hongou liked. The triangles I assume were maybes and the X were dislikes. Others can look at the others but this is just a start to kind of pick the writer's brain.
The Three Garridebs uses the trap door...
Sussex Vampire uses poison and sucking the poison out, also envy is prominent.
The Man With the Twisted Lip Uses doubles (7th actor).
The Problem of Thor Bridge uses suicide and framing others.
Adventure of the Illustrious Client