The first 'grown up' books I can remember reading were my parents' Agatha Christie books. I loved these! Ms. Marple and Hercule Poirot were my heroes. They're a great place for young people to start in the genre of mystery - they're not graphic, they're not violent and they make you think as you try to work out who the killer is right along with Marple and Poirot. So they're appropriate for a younger audience. I think that's why I basically skipped over the Nancy Drew and Hardy Boys type of books - I went from Encyclopidia Brown to Agatha Christie.
If I remember correctly, And Then There Were None was my first Agatha Christie and I quickly devoured the rest. If they weren't in the house, I would scour the library for them. You've got to love those "other books" listing at the front of the paperbacks.
Do you remember the way we used to find books in a library? There were these banks of little boxes that contained index cards. They were filled alphabetically and you'd look through them for your book (so if you didn't know the actual title you were kind of screwed,) and the card would give you the Dewey decimal number of the book and then you'd go find it. Or if you were just browsing, you'd go to the mystery section, for instance, where they were all filed and look at the books themselves, titles on all the spines... I haven't been inside a library in a dog's age. You can even get your electronic books at the library online these days.
So I was a real mystery buff and then I read The Hobbit and fantasy and sci-fi overtook mystery in my reading. Not that I gave the mystery up - I still enjoyed those, but they weren't at the top of my list anymore.
Maybe I ought to investigate - I know there's a bunch of Martha Grimes and PD James books that I haven't read yet... it'd be a great place to pick up again.
smut fixes everything