I'm trying to find a book series done by Edward Gorey. It was illustrated but probably not written by him. It's about two children and their grandfather who go around solving mysteries. It's like a young reader/young adult book series with illustrations by Edward Gorey every 25 pages or so. I don't know the name of the book series and can't make it out from the lists of Edward Gorey's books on wikipedia.

Does anyone know what the series title/ book titles might be?

I'm trying to find these to give them as a gift to someone that told me about them and enjoyed them in their childhood so I'm really wanting to find these books.

Any hints would be great! Thanks!