This is not one of those books you can read without reading the previous ones in the series, because it basically regurgitates what has come before but without much background or any cues of why the characters behave the way they do, or why things happen the way they happen. It is not one in a series. It is one of a whole. You may be able to pick things up here and there because of the conversations (where repetition around past events is a norm), but you won't be able to enjoy much, because this is the prototypical example of a middle book in a series (with the extra that this middle book has been cut in two parts, this being the second).
And what happens? We continue with the preparation for the trial, with all the good and bad this entails. The good: characters get heaps of time to shine, many conversations happen where characters are given the chance to develop and be given a rainbow personality. On the downside it all ends up being conversation after conversation of more and more of the same and you have to wonder why Miyabe decided to extend and stretch the story so much.
If you enjoy the mystery (I hope there is some payoff, because if not... argh) and the characters, you are in for a threat. But better read the three previous volumes before.
The best: the characters are interesting, and the conversations and character development nuanced
The worst: it is another, deep down, unnecessary volume
Alternatives: as said before, when reviewing volume 3, Miyabe's own "Brave Story" is an option, or maybe Riku Onda's "六番目の小夜子". And heaps of manga with a middle or high school setting.
6.5/10
(Japanese)