I've been reading David Bergman's The Violet Hour: The Violet Quill and the Making of Gay Culture, and while I don't necessarily recommend the book as a whole (unless one is extremely interested in the Violet Quill writers, and occasionally aggravatingly academic discussions of the writers' work), there's an intriguing section (pp. 60-67) about Isherwood and A Single Man in the chapter entitled "Gay Writing Before the Violet Quill."
***Spoilers ahead, in case you have not read A Single Man (which you absolutely should!)***
The reason I particularly recommend this discussion is that it takes an interesting, and I think very intelligent, angle on the question of the final scene, which I recall we discussed at great length. Bergman does not think that George dies at the end of the book, but rather that Isherwood is using the playful "let's suppose" of the final two or three pages to both supply and subvert the traditional "the gay man must die" ending of so many prior mainstream gay novels. He also connects the final scene back to the "rock pool" description from several pages earlier, in a way that I wanted to do myself while I was reading but was unable to see my way clear to.
This particular discussion is well worth a look.