I don’t know if you’re familiar with Donald Westlake, but here’s an interesting interview with Brian Garfield about their relationship. Apparently several of the most prominent pulp noir novelists of the era had a long running poker get together. The interview’s mostly concerned with their experiences with the numerous film adaptations of their work and the screenwriting process. I’m almost tempted to check out The Jugger which was allegedly so bad that Westlake sued Godard in court and won. But then post-60s Godard generally has a bad reputation so it’s no surprise.

https://pressblog.uchicago.edu/2011/04/04/playing-poker-with-parker-an-interview-with-brian-garfield.html

If you can find them, the Darwyn Cooke graphic novel adaptations of the first four Parker novels are a good read, and what hooked me into reading him.