I was a huge Nancy Drew fan as a kid, and now own the entire 175 books series. The best ones are the older ones, which are usually hardcover books. Mine are yellow hardcovers, but I believe there was at least one edition that’s older than that. After book 56 they were only published in softcover. The further along the series you go, the further the quality goes down, at least in my view. To tell the truth, I haven’t read the last 40 or so books. I collected them to have the collection, but lost interest in reading them. It was a tradition between my mom and I – she gave me at least one every Christmas from kindergarten to graduation, and then I scoured used bookstores to fill out the collection.
There are also several spin-off series. The Nancy Drew files, Nancy Drew on Campus, Nancy Drew Notebooks, as well as as the Clue Crew which are easy reading chapter books. These are pure twaddle as far as I’m concerned. My six year old has read a few of the Clue Crew, but then asked if I would read her the ‘real ones’.
If you stick to the main series, the older the better, there isn’t anything inappropriate that I can remember.
Joanne