Its called Unicorn blood
with Taekook as the main characters. Description: Jungkook is the young guardian in training ,of all woodland creatures of the Underworld. Taehyung is a thief for hire, a son of Loki.. the Norse god. What happens when he accidentally destroys a treaty that keep the peace?

The second book is called 'The Daegu Train Murderer' this also stars Jungkook but has other  KPOP idols as well.