I read the 1960s versions of the Hardy Boys and The Nancy Drew when I was ~9. I still own most of them. My mother read everything. The house was full of books. At some point over the years having my own family and moving around a bit the books got cut down to only the old stuff from the 1960 that are collectables, stuff from my grandfathers childhood (things with dates ~1900) and hardbacks of non-fiction type things (science, music theory, math, biographies, the classics). This still covers 3 bookshelves. Everything else is pretty much only in digital form. In this lock down I have come to realize how much I miss the physical book.

That outfit, the Stratemeyer Syndicate, has had some good threads on HN over the years:

https://news.ycombinator.com/item?id=18534629 ("The Secret Syndicate Behind Nancy Drew")

https://news.ycombinator.com/item?id=1705714 ("The Hardy Boys - The final chapter (1998)")