Seshun takes Sikue, Tatsuma, and Yokiro on a tour through a rapidly developing town. The modern views are fun and all, but there's new dangers to run into in the murkier parts of town...
This book where we get all acquainted with Seshun joining the party opens the door to SOOO many mini-comics that I recommended against reading due to spoilers.
So yeah, if you've been heeding the spoiler warnings, now's a very good time to go read some of the "Read after book 13" Side Stories!