The Dortmunder and Parker novels employ a loose continuity, i.e. earlier events are sometimes mentioned but you’re not required to have read the books. That said, it’s interesting to read them in sequence for the development of the character in terms of the iterations they go through.
Parker is one huge amoral stone of a criminal who’s only out to take revenge from the go - but gradually we become familiar with him and in spite of his not having any sympathetic traits we are fascinated by his progress and the way he overcomes even the greatest odds, taking on the Mob, the KGB and all kinds of weird characters and bizarre heists. A high mark of the series are the four titles Deadly Edge - Slayground - Plunder Squad - Butcher’s Moon which you might want to read in sequence.
The Dortmunders are actually their own subgenre, comical heist with a Big Apple local patriotism, and they feature a whole troupe of oddball criminals, hustlers, dealers, pushers, specialists for this and that. The first one, The Hot Rock, was adapted to the screen in 1972 with Robert Redford, George Segal, Ron Leibman and Moses Gunn. Due to their nature many weird and hilarious things happen, mostly crushing Dortmunder’s intricate plans and forcing him and his team to improvise.
Here is a little appetiser on Westlake/Stark…
And here is a link to The Westlake Review, a most helpful site when setting out to discover Westlake’s œuvre.