I was going to give this book the same rating as the other three I have read by the same author in the series, but the ending is really weak. The characters' motivations are becoming bloated and comical and Chieko is clearly a villain in an operetta whose secret plan is, or it seems to be, pretty ridiculous. Every time she appears I just hope she has grown a moustache so she can twirl it. Part of the problem is that Mikami En is writing longer plots. If in the two first books there were short stories with interesting characters and change in the scenery, here it all revolves around a little kid and the library his father left him after dying. Also Tobirako is less interesting than Shioriko, in part for being just a copy-paste of her mother. But well, even with the stories not being so well rounded, if the ending had been good... But it is pretty over-the-top and absurd and it relies also on all characters keeping things secret instead of talking clearly.
The best: books and mysteries are very alluring
The worst: Chieko is a silly character; the three stories (particularly the big ending) are meh; maybe Mikami En is not very good at finishing their stories
Alternatives: other books in the series are better than this, and they are pretty easy to read/engaging