"Biburia Koshodou no Jiken Techou" volume 5 by Mikami En, with the subtitle "Shioriko-san to Tsunagari no Toki" (the moment when a person is connected to Shioriko). This subtitle refers to the moment when the two main characters finally advanced to the next stage of their relationship, and it seriously took the entire book to get that far. This slow place is due to Shioriko's analysis paralysis: when it comes to subjects not related to books, Shioriko thinks long and hard before giving her replies carefully. Could be a day later, two months later, or perhaps not until the next volume of this series is published. Shioriko might as well be a lawyer.
Character development aside, the mystery cases involving books were swiftly resolved. Unlike monolithic volume 4, this volume returns to the shorter chapter format of earlier volumes. Many books were referenced in this volume, the highlight of which was probably Tezuka Osamu's "Black Jack" and its many editions. Unusual trivia about old books continues to be the highlight of this series (besides the cover art, which I come to like more and more).
This series is probably near the end, perhaps it will finish before the end of "Hikaru ga Chikyuu ni Ita Koro" series.
Previous (2014-03-21): "Kino no Tabi 17" by Shigusawa Keiichi
Next (2014-04-27): Retro pad