Question #1: should be closed.
Although the question is posted with reference to specific book(s), it's asking about any books which use particular elements:
Have there been any works of literature featuring [...]
has anyone ever made up stories based upon such [creatures]?
This seems no better than the original examples of bad recommendation questions which made us declare them off-topic for this site. I've already voted to close it.
Question #2: not a recommendation question.
This question is asking about the existence of books in a specific franchise, namely Scooby-Doo. It's clearly scoped and will be answerable by a scooby-doo expert. There's no risk of subjective answers proliferating out of control (the main problem with recommendation questions), because it would even be feasible to provide a list of all Scooby-Doo books in a single post and go through them to check which ones, if any, are for adults. It's more comparable to questions like How many Red Dwarf books are there? - strictly contained within a single franchise and therefore reasonably answerable.
The only reason I could see for closing this one is as Primarily Opinion-Based, due to the fact that it can be hard to draw the line between a book for children and a book for adults. But I think a lot of modern fiction, especially in a franchise like Scooby-Doo, is clearly marketed for one or the other, and it's feasible to make that distinction here.