Judging a book by a cover

My mother has accidentally got a book out of the library for holiday reading* that is a Sherlock Holmes/Cthulhu mythos crossover. Apparently the green snaking tentacles and the words "A Cthulhu Casebook" above the title were insufficient information to someone who hadn't heard of Lovecraft and didn't have any particular associations with them. Though unexpected eldritch hasn't stopped her reading it, it is definitely not what she was expecting.

* They are in the Yorkshire Dales for a week, I for a few days. In a terrible disappointment, the mains water here is from a difference source from north Leeds/Harrogate.