The character of Cindy Makey becomes a regular part of the team in the books and Adam's potential love interest, often having clashes with Sally. In the series, Cindy's character traits are divided between Sally and Ann Templeton.
First appearance of Neil and The Howling Ghost.
In narrative order, The Howling Ghost should precede, not follow, The Evil House (released earlier to coincide with Halloween). Here, Adam hasn't joined the team yet, whereas in The Evil House he's the team leader pulling Sally and Watch into celebrating Halloween against their desire and dragging them into an adventure against their better judgment.
Two more Sherlock Holmes references in the series crop up. (1) Watch uses the word "afoot." (2) Mr. Spiney the librarian admires and covets Adam's skull just as Dr. James Mortimer does in "The Hound of the Baskervilles."