Best three spankings in Mainstream non-spanking novels

I'll second or third A Sparrow Falls, Smith also included a spanking description in another of his books THe Sound of Thunder, which was the second in his first trilogy of Courtney novels (A Sparrow Falls was the third). There were references in other books of his, especially in The Sunbird.
There's no description of a single event, but a house slave in one of Kyle Onstott's books by the name of Lucrezia Borgia spanks her twins on a daily basis and it's suggested that she even spank grown male slaves as punishment.
Robert Jordan sprinkled spanking references throughout his Wheel of Time epic fantasy series, and there were so many in A Knife of Dreams that I think it should have been subtitled 50 Shades of Wheel of Time.

My Sweet Audrina by V.C. Andrews contains an excellent description of the OTK spanking of a late teenage girl.

They made a TV film of the book in 2016 which I have never seen, but I expect they omitted the spanking.

There is a rather nice strapping of a young lady's bare bottom by another woman in Richard Adam's "Maia".

Robert Nye (1939-2016) was a well-respected English novelist and poet, but obviously also a spanko. I have read three of his novels, "Falstaff", "Merlin", and "Faust", and they all contain spanking scenes written with gusto and glee. "Falstaff" may be the best novel (it won the Hawthornden prize), but in my opinion "Merlin" has the best spanking scenes. And I think I remember a line from "Faust" verbatim: "Never leave a girl of seventeen half-spanked."

I would also like to mention my compatriot Knut Faldbakken who put a very interesting and quite intense M/F spanking scene into his novel "The Honeymoon" (English translation available). He has been out of grace with Norwegian feminists ever since, but that is really their fault and not his.

A Spy in the family by Alec Waugh, a well written, literate and erotic book which is largely about spanking.
He is one of the three Waugh brothers, best known was of course Evelyn, the other Auberon Waugh.
Certain parts of the book, which is very much English public school (v expensive private) makes me wonder if his spanko pseudonym was in fact Hilary Chale who wrote a number of fairly severe spankng books which are available on Kindle.
A Spy in the Family is regrettably long out of print.

Alec Waugh was Evelyn's brother but Auberon was Evelyn's son.

Alec Waugh also had a charming F/m spanking in The Fatal Gift (1973)

I think it is an exaggeration to say that A Spy in the Family (1970) is all about spanking although there is a good scene where a wife canes her husband in the bedroom. Alec Waugh described the book as an erotic comedy. From memory I don't recall that it is particularly -if at all - English public school.

His public school novel, a succès de scandale due to its heavy hints about homosexuality, was the Loom of Youth (1917) and this does have several references to M/m canings.

Thanks for the correction re Auberon.
I perhaps used the description English public school, incorrectly. I intended it as a shorthand for saying that the main characters were from the English elite, ie moneyed background, good public school, Oxbridge and a career in the upper echelons of the Civil service.
I shall never forget this book, I first read it when looking through a pile of my dearly beloved late father's library books-he used to go to the public library every week. I took it out for myself later and very many years later was fortunate enough to buy a reasonably priced secondhand copy.

Rather than three books it's two prolific authors. All these books contain multiple spankings or spanking references.

R.F. Delderdield's historical fiction spans the end of the 19th century up to the mid-20th. My favorite Delderfield spanking is in THE SPRING MADNESS OF MR. SERMON. A woman describes to her daughter's long suffering 49 year-old husband how she used to spank her. Mother-in-law suggests that he should try it. Later in the book, he takes his mother-in-law's advice, spanking his wife on her underpants with his hand and a hairbrush.
Delderfield, R.F., THE AVENUE GOES TO WAR (1940-1947), Simon and Schuster, 1958.
Delderfield, R.F., A HORSEMAN RIDING BY, Simon and Schuster, 1966.
Delderfield, R.F THE SPRING MADNESS OF MR. SERMON, Simon and Schuster, 1963.
Delderfield, R.F., THEIRS WAS THE KINGDOM, Pocket Books, 1972.

Sharon Green has an exhaustive list of books but these fantasies all contain spankings. The willful heroine often finds herself spanked by her lover. My favorite scenes are in the WARRIOR books which contain barbarians from another world and a space traveling woman who receives several spankings and switchings from her barbarian protector whom she can't admit she loves.
Green, Sharon, CHOSEN OF MIDA, DAW, 1984.
Green, Sharon, GATEWAY TO XANADU, DAW, 1985.
Green, Sharon, HELLHOUND MAGIC, DAW, 1989.
Green, Sharon, JALAV IV, THE WILL OF THE GODS, DAW, 1985.
Green, Sharon, LADY BLADE, LORD FIGHTER, DAW, 1987.
Green, Sharon, MIND GUEST, DAW, 1984.
Green, Sharon. THE WARRIOR ENCHAINED, DAW, 1983.
Green, Sharon, THE WARRIOR REARMED, DAW Books, 1984.
Green, Sharon, THE WARRIOR VICTORIOUS, DAW Books, 1988.
Green, Sharon, THE WARRIOR WITHIN, DAW Books, 1982.

Susan Isaacs' Almost Paradise describes the female protagonist's upbringing at length, which included realistic, utterly loveless spankings that played a role in the heroine's inability to form a close bond with others later in the story. Not the kind of thing I find erotic (I'm more disturbed by cruelty and lovelessness than by actual harshness) but definitely a realistic series of spanking descriptions in a vanilla book.