The relationship between Bukowski and Martin would make an interesting book. A collection or analysis of their correspondence. You can see in many of Bukowski's letters that he tiptoed around Martin most of the time, when not kissing his ass outright. I don't know if Martin needed that or Bukowski was just insecure, but their relationship was unlike any other relationship in his life.
He knew which side his bread was buttered on, and he was thankful for Black Sparrow, but you could tell that he often bit his tongue when he would have let anyone else have it. After he started seeing more income from non-Black Sparrow royalties did you see him question Martin more frequently. Even then, though, it was mild compared to how he could be with others.
He was usually polite to the Webb's (and other publishers too) in letters to them, even when he was angry or frustrated with them. But he complained about them to others, which he didn't do very often where Martin was concerned.
This is just a top-of-the-head read based on what I remember from letters. But a collection of all their correspondence would be very interesting to read. And probably disprove everything I just typed, who knows.