3.5 stars. (Update: I'm rounding up to 4 because the stories are sticking with me over time.)
I'm sure someone else has already called this, and I am just inadvertently repeating it, but: Chick Lit as written by a suburbanized Dorothy Parker or Anita Loos (and please know these are writers I hold in very high regard). Hold on to that Dorothy Parker reference, as I will be returning to it when I discuss the book's redeeming qualities... after this brief message.
This is a book of short stories in which Most of the protagonists are fundamentally or habitually self-centered, educated, young, attractive, upper-middle-class, suburban white women who are married or about to be married (usually to a mild-mannered, nice, clueless guy who works in a tech firm or economics); have children or are about to have children; and, if not yet full-time mothering, may work for about twelve hours a week in a bookish or writerly kind of job. Oh, and lest I forget: as noted in many other reviews here, pretty much all of the women are having affairs (usually with a mild-mannered, nice, clueless guy who is also married, a little older, and works in some bookish or academic kind of job).
I see that many other reviewers are disconcerted by all this rampant affair-having. Many of the reviews decry the book for supposedly advocating affairs without consequences. Now, I think there are many legit reasons to not love this book, many of which could relate to the omnipresence of homogenous main characters blithely having homogenous extramarital affairs. However, I think the suggestion that the book advocates such affairs, just because the main characters don't necessarily get caught or punished, is as silly as saying that murder mysteries advocate murdering.
Now, the book may more justifiably offend a reader with its occasional repetitiveness and shallow characterization. Seriously: the main characters are literally indistinguishable from one another. Sometimes the same main character reappears in several stories, and sometimes the main character is different; it honestly does not matter because they are all exactly the same person. And they really are All Always Already having affairs, to the point where the affairs don't even seem to be the point anymore and kind of become background noise. Like, the car alarm can only go off so long before you tune it out.
Because everyone's having an affair from story to story, it stops being really interesting or notable, except for to note that you have to admit it's different to have a book where the affairs are Not the focal point or a big effing deal. Especially if it's women having the affairs! I guess this was somewhat refreshing, if you want to put a positive spin on it, but I understand some may not want or be able to do that, so - let's move on.
So just assume the affairs are just happening left and right throughout these stories, and that the stories may not necessarily be "about" these affairs, at least not entirely. So what else is going on? Well, admittedly a couple of the stories do show the shared protagonist's modest personal growth and maturation at various snapshots in time. Several of the stories have touching, humorous characterizations of being a mother in the age of expected perfection and professional mommyhood. There are some good, realistic (and emotionally healthy!) depictions of female friendship and fellowship (including with oft-maligned women relatives, like mothers-in-law). Some stories feature neighborhood cul-de-sac relationships and power dynamics: HBO's Girls meets Desperate Housewives.
Is all of this enough to raise the book beyond the level of typical chick lit, or a sitcom, or a really good Super Bowl commercial? Maaaaybe, maybe not, but - here's where we get back to Dorothy Parker. Heiny is a really good writer with a unique, witty, concise style and voice that kept me reading. (With regard to the concision - yeah, it read sort of like Hemingway, only as a woman, not intoxicated, and with a sense of humor, writing present-day, high-caliber chick lit.) How good this first book is or is not, I'm not sure, but I do know for sure that Heiny has writing talent and that I'd happily check out more of her future work to see where else she can go with it.
A word about the (great) title: Is it ironic? Does it have a double meaning? Is it a mischievous clue as to what Heiny is trying to accomplish? Maybe in part. The characters in the stories are single, carefree, and mellow in that (as the unhappy reviewers noted) they behave in an unfettered way, do what they want (which seems to be have affairs), and aren't even necessarily wracked with guilt about it. However, the title also seems ironic in that the characters are mostly Not single, and also neither carefree nor mellow in that they don't seem especially happy or guilt-free either, despite their attempts to embrace hedonism. There is definitely an overarching tone of (damned if you do, damned if you don't) nihilism here, which seems very Modernist and hence reminiscent of writers like Parker and Hemingway. However, if you can get to the end of this book, the characters do seem to be -- slowly and often misguidedly -- but ultimately -- trying to resist this overarching meaninglessness and to gain insight into self/other and truer connection to others. Which is a human condition worth reading about.
The phrase "Single, Carefree, Mellow" is also both the title of and a key phrase in what I found to be the best story in the book, a story that also notably achieves the rare and accomplished feat of featuring an aging dog in a way that is tearjerking but does not seem exploitative or like a cheap and easy unearned shortcut. I would also recommend "That Dance You Do" as one of the funniest stories and best "motherhood" stories; "Grendel's Mother"; and "The Rhett Butlers."
You just have to give this one a read to see whether you like it! It's that kind of book.