The book wasn't bad, but it was the worst of the series so far. The problem is that the whole investigation doesn't lead anywhere until the very end. In fact, the culprit is revealed to be the culprit in the last 20 pages and only then it is explained how one of the main characters found out everything. Also, there's one detail about one of the secondary characters that is extremely obvious, but Irene, Sherlock and Arsène don't realise it until the end. They should have at least suspected it much sooner.
