Las sombras del Sena - Irene Adler
I didn't find the mystery particularly interesting. However, there are things hinted in this book that are much more mysterious than the case that the three friends (or four, as Horace decides to help them instead of trying to stop them) solve. Horace's identity is the most interesting. We don't get all the information about him, although I hope that the next book sheds some more light on this character, but it becomes clear that he is more than just some man who works for the Adlers.
Even though the case is not very engaging, this book has a much more mature feeling than the previous five, especially towards the end, when the three teenagers realise that the case may be a bit too much for their age.
It is a book worth reading if you like this series, but don't expect something extraordinary.