Bungo Stray Dogs, Vol. 2 (light novel)

Before joining the Armed Detective Agency, Osamu Dazai was the youngest executive with the Port Mafia, the most notorious underground crime syndicate in all of Yokohama. When one of their colleagues suddenly goes missing, Dazai and fellow Port Mafia member Sakunosuke Oda are tasked with conducting an investigation...and at last, the reason why Dazai parted ways with the Port Mafia is revealed!