> I read some short mystery stories by Alfred Hitchcock once that were good. But I think those were written for young readers too. I'm not sure what he's written for older audiences that hasn't been made into a movie.

I could be wrong, but I suspect those were "edited" and "presented" by Alfred Hitchcock. He's had several short story anthologies published under his name (books and his Alfred Hitchcock Mystery Magazine), but I don't remember ever seeing anything he wrote himself. Hitchcock was a great director, but the only significant writing work I know of him ever doing was title and script work in the 1920s and early 1930s.

However, the authors he adapted and featured most often, including Cornell Woolrich and Daphne du Maurier, are fantastic authors and can be enjoyed regardless of age.

The mystery series that Hitchcock "presented" in competition with the Hardy Boys -- The Three Investigators, conceived by Robert Arthur and written by other authors after Arthur died -- are, IMHO, much better than the Hardy Boys. The plots tend to be more diverse and colorful; of course, some are better than others. After dozens of series entries, the Three Investigators hit a lull and were later revived as a new Three Investigators series that had the kids a bit older ; the books had more of an edge to them. I never really dug that second series, but I was starting to move on at that point anyway.