"Kaijin Nijuumensou" (Phantom of Twenty Faces) by Edogawa Ranpo. This was targeted for a younger audience and was very easy to read, even comes with illustrations. The plot is very linear and fairly predictable, and readers never really need to remember much context to follow along (unlike Ranpo's more complex stories, such as Nisen Douka). Despite the simplicity, the overall fast pace of events and the boldness in how the characters acted made this quite exciting to read. Also, although the cover and the illustrations may look retro, the text does not feel old at all.
This was the first novel in the Shounen Tanteidan (Detective Boys) series, I am not sure if I would read through all remaining 30+ novels, but there are definitely a few more that are worth reading.
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