The collection of Kurt Vonnegut’s letters is Here . They’re wonderful, and well selected, and provide a fascinating path from one end of a life to the other.
You make an interesting point about Vonnegut's "anvil" and that he had his limits with it.I have recently begun "testing" some of the typewriters in my collection for Writing Longevity, i.e. seeing how they feel after typing many pages of text. I am somewhat disappointed to find that many of them stop feeling comfortable to type on after 8 pages or so.As for Salinger, I suppose it is better to have written one wildly best-selling novel than none at all. It sounds like he was able to survive for many years after "Catcher"'s publication.
I know. There's no arguing with success, and all of that. It's just a shame that he spent so much of his life alienating all the people his work meant so much to.