Bungo Stray Dogs, Vol. 1 (light novel): Osamu Dazai's Entrance Exam (Volume 1)

Doppo Kunikida is an idealist and a straitlaced detective at the Armed Detective Agency, an organization that takes on dangerous jobs even the police won't handle. Everything in his life is going just as he's planned...until one day, he's paired up with the agency's newest hire: a suspicious, eccentric, suicide-obsessed man named Osamu Dazai. Their first case together turns out to be far more complicated than Kunikida anticipated -- and it looks like the detective agency's sworn enemy, the Port Mafia, is somehow involved, too!
Set two years before the events of the manga series, this is the tale of how two seemingly incompatible personalities end up forming the partnership that has made them one of Yokohama's top detective duos!