Now that I recall it, I fell into Westlake/Dortmunder because of a DC project pitched at me by Andy Helfer that never got off the ground, sometime back in the very early 90's, it was a would-be JLA spin-off featuring some villain-types in a Dortmunder-style series of misadventures. I bought the books and the job never materialized, so I was out about $20, which I honestly couldn't afford at the time. Oh, well, then as now, ramen was dirt cheap (and about as nutritious), and the books were swell.
ADDED: Now that I further recall it, I think my first encounter with Westlake/Dortmunder was when Jim Hanley lent me a copy of one of the Dortmunder paperbacks while I was spending a night on the couch at Jim Hanley's Universe, after yet another row with my parents. I know, this is all very fascinating.
I got into the Parker material only a few years ago, after talking it up for years the Lawgiver finally just gave me a few of the early volumes for a gift. I thought they were great. I still have a number of Parker books to wade through when time/availability allows, iirc they were not all in print or still aren't for whatever reason, or were at least difficult to track down a few years ago.
Westlake's one of those guys I just figured would be putting out another twenty books before he kicked. He certainly had a great run, though.
Anyway, R.I.P. to him, and all the pseudonyms, as well.