Shadowing falls under this category, and you might like it if you have read and liked any of the following. And if you haven't read them, you should try at least one.
All too often in fantasy books,dispatched with a lazy flick of the hero's sword. But in Shadowing, the job benefits of fire, mayhem, and flight almost make up for it. When a man in golden armor and his faithful companions sneak across the border on a quest to assassinate , it's up to the Shadows to cut his quest short. As always, things do not go quite according to plan, and one henchman ends up lone-wolfing it across the permafrost lands, following the stench of valor.
A light picaresque involving a retired assassin, demons, demigods, and glass butterflies. The assassin inherits his cousin's caravan, and makes a dangerous journey to a city by the sea. Assassination attempts at every turn. Witty dialogue and other good things. I have fond memories of a very unconventional wizardly battle involving name calling and talking smack.
Eve Forward - Villains by Necessity
This is out of print, so good luck getting your hands on a copy. I hope one day it will be ebooked, because it was one of the more amusing fantasy books I've read. Samalander, the last assassin in the world, joins a neutral druid, a black knight, an elf sorceress, a centaur, and a thief. Together, these cooperation-haters make up the last gasp of Evil in this Good-conquered land, and only they can restore balance to the world.
There are few books that manage to find just the right blend of humor and badassery, but Brust's Vlad series often hits it well. Vlad the assassin is an underdog in a criminal underworld. I hesitate to class it as fantasy, because there is little if any walking in real time across a field, but there are other species and magic. Early books in the series make good use of organized crime, and have something in common with classic noir as well as sword and sorcery. Taltos and Dragon were my favorites.
Roger Zelazny - Nine Princes in Amber
Tarot cards, amnesia, patterns, and court intrigue delivered through a sarcastic narrator. Perhaps this doesn't quite fit the list with a prince as main character, but the whole family is immoral enough that I think it counts. And besides, everyone ever is out to kill Corwin. But if you want Zelazny and distinctly not high fantasy, try A Night in the Lonesome October, which is told from Jack The Ripper's dog-familiar's perspective.