The Mystery of the Brass Bound Trunk by Carolyn Keene. Gorgeous cover art by Russell Tandy.
The Message in the Hollow Oak by Carolyn Keene. Artwork by Russell Tandy.
The Clue of the Broken Locket by Carolyn Keene.
The Secret at Shadow Ranch by Carolyn Keene.
The Dana Girls. By the Light of the Study Lamp by Carolyn Keene.
The Dana Girls. The Ghost in the Gallery by Carolyn Keene.
The Dana Girls. In the Shadow of the Tower by Carolyn Keene.
The Hardy Boys. The Secret of the Old Mill by Franklin W. Dixon.
The Hardy Boys. The House on the Cliff by Franklin W. Dixon.
The Hardy Boys. Footprints Under the Window by Franklin W. Dixon.
Beverly Gray's Vacation by Clair Blank.
Beverly Gray's Adventure by Clair Blank.
A Judy Bolton Mystery. The Midnight Visitor by Margaret Sutton. Probably illustrated by Pelagie Doane.
A Judy Bolton Mystery. Seven Strange Clues by Margaret Sutton. Probably illust. by Pelagie Doane.
A Judy Bolton Mystery. The Yellow Phantom by Margaret Sutton. Illustrated by Pelagie Doane.
A Melody Lane Mystery. Terror at Moaning Cliff by Lilian Garis. Illustrated by Ruth King.
A Melody Lane Mystery. The Tower Secret by Lilian Garis. Illustrated by Ruth King.
The School on the Moor by Angela Brazil.
Two Girls and a Mystery by May Hollis Barton.
Jimmie Drury Candid Camera Detective, by David O'Hara.
Cherry Ames Chief Nurse by Helen Wells.
Vicky Barr series. The Mystery of Flight 908 by Helen Wells.
Jane, Stewardess of the Air Lines by Ruthe S. Wheeler.
Five Go to Smuggler's Top by Enid Blyton.
The Sea of Adventure by Enid Blyton. Cover art by Stuart Tresilian.
The Rockingdown Mystery Enid Blyton
The Outdoor Girls in the Air by Laura Lee Hope
I like to say that I jumped from Nancy Drew to Agatha Christie because that's the way I remember it. But I know I also read The Dana Girls (which I loved even more than Nancy) and likely there might have been a couple of other series - now faded from memory.
All I know for sure is that I loved the titles, the early covers, the artwork, the end-papers and of course, the adventures which showed how young people could do for themselves and take charge even way back then, all the while solving mysteries which baffled the rather ordinary police. For sure, it was fantasy, but I fell hard for it all.
I can't remember reading any Enid Blyton, but I'm thinking it's never too late. At one time Blyton was a beloved and widely read writer whose books were in every kid's library - maybe even mine.
You can read more about these various series at this providential link.
Eventually and with regret, I outgrew Nancy and her friends but I know that these books instilled in me a love of mystery fiction (and reading) which continues to this day.
And I know I'm not the only one.
P.S. Wanted to add that my original link to download the Blyton books for free no longer applies. Apologies for that. But I did download the first of the The Five adventures for 99 cents over at Amazon. You can also download various others for 3.99. At least for now. Of course I'd rather have the actual books with the oh-so-charming covers but needs must.