Patricia Windsor has some really good ones. I thought The Christmas Killer was excellent but all of hers are good. Well written and exciting, touch of the supernatural.
Nancy Werlin's The Killer's Cousin was also good. A young man is staying with his cousins after getting out of jail for setting a fire in which people died.
Joan Lowery Nixon wrote lots of mysteries and you may like them. I found some to be good but some. . . not so much. I found myself saying, "Oh, please, like THAT's going to happen!" The earlier ones were better.
Lois Duncan-- have you read Killing Mr. Griffin or I Know What You Did Last Summer? Please don't judge the books by the movies! She did several other really good ones too.
I always like John Bellairs, too. One of his main characters is always a little young (pre-teen) but there's an older adult and a good creepy mystery. I think the first one I read was Curse of the Blue Figurine. No cop-outs, either-- none of that "Oh, the ghost was just a bedsheet" sort of thing.
Have you tried any of the Alfred Hitchcock collections of mystery short stories? Some REALLY good ones in there, too.
Hope you find something good!
