Contrast with Heinlein on writing: "Writing is not necessarily something to be ashamed of, but do it in private and wash your hands afterwards."
I can't remember a case where he spoke or wrote about writing in a similar fashion to what C.S. Lewis wrote in his letter, but his 5 rules of writing[1] have been influential for many writers. Once, in an Annapolis address, he explained that the rules were simple (1. You must write. 2. You must finish what you write. ...) but only 1 in 10 would follow each step. This was how you could get from half of the adult US population wanting to be a writer to less than a thousand actually being members of the writing guild he belonged to.