Here's a poll asking for your favorite young detective.
I voted for Nancy Drew - I don't think the Nancy Drew books hold up for an adult reader, but I still remember the thrill of finding a title I hadn't read in the library. And my beloved aunt Emily would often surprise me with a new book, which was the best treat ever!
When I look back on those books, it's the titles I remember most. They were so evocative - exciting without being terrifying. And I still remember a lot of the words I had to look up - bayou, coupe, hearth, pavilion - although it was years before I learned to pronounce those words correctly.
There was something so intriguing about THE SECRET OF THE OLD CLOCK, THE HIDDEN STAIRCASE, THE CLUE IN THE CRUMBLING WALL, THE CLUE OF THE LEANING CHIMNEY, THE SIGN OF THE TWISTED CANDLE, THE MOONSTONE CASTLE MYSTERY (which inspired an interactive story, a continuing saga, with my best friend and a heroine called Lillian P. Potter. Yes, Harry Potter's mom-to-be! Little did we know...) and so many more. Just going through the titles makes me want to read them again.
I liked Trixie Belden and her friend Honey a lot, too. Also Judy Bolton and occasionally Cherry Ames.