Auchincloss is primarily a short story writer. In his introduction to his Collected Short Stories, he genially says that he thinks some of his best work is in the short form. But in fact even his most famous novel, The Rector of Justin, isn't a long form story so much as it's a series of shorter works, with the main character the connecting thread.
It is extemely interesting that Auchincloss once had a bestselling book, with Justin. Nowadays, to the extent that one thinks of him at all, one sees him as a nice grandfatherly type relegated to the back pages of The New Criterion, sipping sherry and writing notes on Edith Wharton. Hard to imagine Auchincloss as young. My copy of Tales of Manhattan had a picture of Auchincloss with presumably his kid. It was forty years ago and he looks old there.
Auchincloss is plowing the same field that Marquand did, and many others have -- "the decline and fall of the gentry", with "the gentry" being variously defined. Marquand was obsessed with the reality of the notion itself; Auchincloss more or less takes the decline for granted, as a jumping off point. This gives the writers varying strengths and weaknesses. Marquand meant it so damn hard that much of his work has the humorless thudding feel of a tract.. Yet when it works, as I think it mostly does in Point of No Return, it has an undeniable power. Auchincloss is a much better, much more consistent line by line writer than Marquand ever was, and his work has a sprightliness,, a bounce and a sense of humor that's often sorely needed in Marquand. On the other hand, a lot of Auchincloss feels like a genial waste of time. The poor old duffer would probably keel over if I told him this, but a lot of his work feels to me like very highbrow Judith Krantz. It has the same sort of voyeuristic peek into the lifestyles of the rich and famous, the same sort of "secret sharer" mentality that pervades a lot of this kind of writing. Of couse, Auchincloss is more acerbic and ironic about it than Krantz, but I'm not really sure that equals out to "depth", Henry James references or no Henry James references.
So I think he's basically skippable. You're not going to get much bang for your buck here. Auchincloss is usually pretty good with scene-setting, and he usually tosses in some interesting reflections about character along the way, but I don't know if that makes up for actually having to plow through The House of Five Talents, say. Again, it's interesting Auchincloss was once a bestselling author. Times were certainly different in the mid Sixties.
What's really needed is a sympathetic editor who can go over Auchincloss's complete work and pull out the best bits. If there was ever an author who's screaming for one of those "Portable" anthologies, it's Auchincloss. As I said, even his novels are really short story cycles in drag, it's not like you're going to miss much by way of context. In the meantime, if you really must, The Rector of Justin is easily available and fairly representative of the kind of thing Auchincloss does. Charting the life of the founder of an exclusive private school, it is quite readable, very evocative, and often very wise and knowing about humans and their frailities. One does finish it, though, wondering what the hell all the fuss and bother was for.