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I found this to be a fun read. It's not too difficult and as a children's novel has furigana.
What makes this rather interesting is the scientific nature of the events. Not only does the titular character explain them in the story, but there are little summaries at the end of each case. These briefly explain scientific concepts and important people, making a nice change from just a simple story.
I'd say the main story is similar to きまぐれロボット in difficulty, but those extra explanations push it just above that book.
An N3+ (or prior to the N2 test) student should find this to be a comfortable read without too many lookups.