Grosso modo : meh.
Mlle Mathilde Stangerson is attacked at night in her yellow bedroom and the perpetrator magically escapes without anyone seeing him. Her room is literally locked and there is no other means of access. Enter Joseph Rouletabille, the 18-year-old reporter who aims to beat the police at their own game and unravel the mystery. And for such a set-up, the mystery turned out to be surprisingly weak.
I think this book would have been more interesting if there hadn’t been the layering of multiple written accounts. It did serve to set us back in time, yes, but it also served to delay the unfolding of the narrative because there were multiple explanations and asides where new documents were used. Furthermore, there were several artificial delays where the narrative interrupted itself to postpone revelations in what I can only describe as a gauche manner. By the time we were allowed to unravel the mystery, it was a bit of a let-down. There was so much re-hashing that it felt far too drawn out.
I also found the arranging of people just-so in the middle of the night in order to catch the intruder to be ridiculous.(show spoiler)
So if you’re looking for a better French mystery novel that was published in the same year, go read Arsène Lupin, gentleman-cambrioleur by Maurice Leblanc instead. It’s a lot more fun. It’s also available on Gutenberg in English and French for free.