Bungo Stray Dogs Side Story: Yukito Ayatsuji vs. Natsuhiko Kyogoku (light novel) by Kafka Asagiri

A brilliant investigator known as the "man-slaughtering detective," Yukito Ayatsuji has an ability so deadly that he's assigned a minder, rookie Special Division agent Mizuki Tsujimura, to monitor him. And when Ayatsuji is tasked with solving a murder case, who should be pulling the strings behind the scenes but his archnemesis...Natsuhiko Kyougoku?!
Binding: Paperback / softback