Bungo Stray Dogs, Vol. 2 (light novel): Osamu Dazai and the Dark Era (Bungo Stray Dogs (light novel), 2)

Before he joined the Armed Detective Agency, Osamu Dazai was the youngest-ever executive with the Port Mafia, the most notorious underground crime syndicate in all of Yokohama. Three of its members-Dazai, Odasaku, and Ango-formed a camaraderie despite their vast disparities in rank. When Ango suddenly goes missing, Odasaku, a lowly Mafia grunt, receives direct orders from their boss to find him...only for Ango's trail to lead Odasaku to a mysterious, foreign criminal organization called Mimic.
At last, the reason why Dazai parted ways with the Port Mafia is revealed!