This book is a collection of short stories regarding an investigator's office with only two employees: the investigator himself and his staff assistant. The main investigator is called 最後の鑑定人, the last appraiser, since he's the "last" appraiser people go to after exhausting all other options because of his competency. In each short story, the investigator's office work on usually severely violent cases, utilizing different investigative methods to finally get the criminal to confess to their crime by cornering them.
It's a hard read. It's ultimately about people who have committed horrible acts, and seeing how they try to defend themselves or basically act like everything they did wrong is someone else's fault. It's not really about the inner machinations of the psychopathic criminal mindset like some media (in this book, the criminals will often try to mess up the confessional narrative by insisting they were totally panicked, totally unaware of what they were doing during the actual act, while trying to normalize their behavior as much as possible) but it's strangely striking to see how every criminal gets stripped down to a pitiful thing in front of the investigators after ruining other people's lives, sometimes in the worst possible way. I don't think this book will get translated into English but I wonder if anyone will do a translation of the live-action show that's apparently out there, though I haven't watched that yet.