"Biburia Koshodou no Jiken Techou" volume 4 by Mikami En. Subtitle this time is "Shioriko-san to Futatsu no Kao" (the two faces of Shioriko-san). This appears to be an obvious reference to Shioriko's mother, but actually there are a few other more subtle two-faced characters throughout this book. There were more surprises than expected (though generally, that's how surprises are supposed to work).
Unlike the previous volumes, this volume is a single long story focused on a single author: Edogawa Ranpo. Thanks to the extra length, and also because Edogawa Ranpo was a fairly popular and prolific author, there were more historical details and anecdotes compared to previous volumes. This book should be very appealing to Edogawa Ranpo fans, especially those interested in the mystery side of things as opposed to the abnormal personalities. I was very happy with how some of Ranpo's devices and puzzles were reused.
With the story focusing more on a single author for the entire volume, and with Shioriko's mother's role becoming more prominent, this series is feeling more and more similar to the "Bungaku Shoujo" series. I think this is a great direction to go, especially if this series would continue to reveal more interesting things about books and authors.
Previous (2013-08-29): "Biburia Koshodou no Jiken Techou" volume 3 by Mikami En
Next (2013-10-31): "Codex Seraphinianus" by Luigi Serafini