Geheimnis Am Holunderweg without using the dictionary once, and it seems to be aimed at preteens or young teens. For purposes of improving my German, reading this book was light years more helpful than my recent slog through Kafka, in which I had to look up every other word, which caused me to frequently lose the author's train of thought. There were plenty of words in this book that I didn't know, but the simple sentence structure and story line made it easy to figure out meanings from context, without slowing down.
The edition I read was printed in 1970, and set in a small German town. (Holunderweg is the name of a street, and the title translates as The Secret of Holunderweg.) I initially assumed that this was a translation of a work by the famous English children's author Enid Blyton, but the German setting made me wonder, and so I did some googling. Although I couldn't find any reference to this title in particular, apparently there were many books, and series of books, written by French and German authors in Blyton's style and using her name, either with or without permission. This would seem to be one of those.
In the course of my googling I also learned that many of Blyton's books were full of racial and gender stereotyping and later editions were usually edited to remove insulting references to blacks. And apparently in many of her books in which groups of children had adventures and solved mysteries, girl characters were excluded from the more dangerous events, and always did the cooking/cleaning up for the boys. This issue did come up in Geheimnis Am Holunderweg. At one point the oldest boy tells the two girls that they may not come along on a midnight search for some stolen goods because it would be too dangerous for them, and the girls willingly comply. I don't know which seemed more anachronistic to me--girls happily taking orders from boys, or twelve-year-olds running all over town unsupervised day and night rather than being driven to activities and play dates by their parents!