"Biburia Koshodou no Jiken Techou" volume 3 by Mikami En, with the subtitle "Shioriko-san to Kienai Kizuna" (Shioriko-san and the unbreaking bonds).
The common theme running through all chapters for this volume are the various family ties, including the link between Shioriko and her mother. There is an increased of plot direction for this series, possibly ending with Shioriko's mother as the final boss or mastermind.
Besides the progressing story, there is the usual set of mysteries that are solved using knowledge of old books, which are still the highlights of this series. So far, this series has been totally worth reading, at least for the benefit of learning about other books that I might also want to read. The chapter on Miyazawa Kenji was especially rewarding for me, but I think I will continue on with volume 4 of this series before resuming my book buying spiral.
Previous (2013-08-23): "Ochibo hiroi / Inu no Seikatsu" by Koyama Kiyoshi
Next (2013-09-10): "Biburia Koshodou no Jiken Techou" volume 4 by Mikami En