Something like .trash or voynich hotel or the SOS Brigade I also forgot the title of this manga that's like .trash who has teenager assassins but disguises as call girls
violence action by sawada shin... it's good but updates are sloooowwww.... edit: it's the manga you're looking for I guess... It has assassins disguised as callgirls.
Not a manga but the game 'Vampire: The Masquerade – Bloodlines' always gave me that cool underworld secret vampire society kinda feel....and it's got the best damn writing I've ever seen in gaming.
Hinamatsuri It setting is in modern yakuza world. Check out vol 7 chapter 32. Realistic description of journalist, risking his life investigating yakuza world. Chapter 32.5 show the end result.