I've loved reading the Parker series. They're much darker than the Westlake Dortmunder books, which are comic crime caper novels.
If you're a Parker fan, the first Parker novel (The Hunter) has been turned into a movie no less than three times:
The Hunter (adapted three times, as Point Blank (starring Lee Marvin) in 1967, as Full Contact in 1992, and as Payback (starring Mel Gibson) in 1999).
As it happens, I just watched Payback on TV a couple of days ago-- it's a really good adaptation.
If you've seen any of those movies & enjoyed them, you should check out the novels!