Crime fiction writer Dennis Lehane is an author who is excellent at writing about moral ambiguity. He has a series that starts with A Drink Before the War [Harvest Books 2003, 300 pages] that features private detectives Patrick Kenzie and Angie Gennaro. The series is often dark and violent but with careful use of humor, the author manages to lighten things up a bit. There is a romantic thread in here that is nice so romance fans would enjoy it that is if they can withstand the violence. If you enjoy character driven mysteries then I recommend this author highly.
In reading order:
A DRINK BEFORE THE WAR
DARKNESS, TAKE MY HAND
SACRED
GONE, BABY, GONE
PRAYERS FOR RAIN
MOONLIGHT MILE*
Standalone novels:
SHUTTER ISLAND
MYSTIC RIVER
THE GIVEN DAY
LIVE BY NIGHT (Oct 2012)