Identify a book, love triangle, death of a woman, no Personal Computers

Question

I have read a book in the 90's, while I was a teen, but it is very likely it was published much earlier. I have been trying to find it on and off since. I do not remember character names. Sorry. Things I do remember:

• The book is of novel length
• Technology level is middle 20th century. No magic. No personal computers.
• Point of view character is a man.
• The book starts with the man going to huge property (with a house) of a millionaire, who is his friend. Millionaire became millionaire by revolutionising steel industry, I think.
• It is possible that they are friends from after the war. I think WWII but I might be wrong.
• While a driver or his friend is driving him to the house, the man sees a tractor ploughing the field. The tractor has no driver; it is tied with steel cable to a steel or iron boulder and is going by itself in ever shortening circle as the cable winds up. The millionaire explains that the tractor is one of his automation experiments; he wants to revolutionise agriculture the same way he did steel industry. That means circular field which can be worked on with automated tractors, with houses and storage on the edges of formerly square fields. That's the only sci fi part of the novel.
• The millionaire has a beautiful wife. She loves him very much, but is something of a wild spirit, a force of nature. She likes to ride horses, she likes speed, is a good swimmer etc. Sort of like Teddy Roosevelt in female form, if that makes sense.
• I remember a scene near the start of the novel. The man is introduced to wife, there is a pool party after a while and millionaire friend tells him that she can hold her breath for 20 minutes or something like that. There is a bet involved, I think. The wife dives in and does not dive out. The man sees that nobody at the party is worried so he realises it's a trick of sort. He dives in too, multiple times and she is nowhere to be found in the pool. So he dives in one last time and finds a hatch to secret underwater chamber where there is breathable air and she is inside. It might not have been pool, maybe some small (artificial) lake on the property.
• They fall in love, the man and millionaire's wife. I am not certain if they ever slept together, but they were deeply in love. The man feels bad about messing with his friend's wife but the millionaire is not that much conflicted although it is very tricky situation. They are two people he loves most in the whole world, one as his best friend and one as his wife and lover. He is intelligent man, a genius if you will; he doesn't want to ruin friendship and marriage just because his best friend and his wife love each other. I think that he even jokes at one point, something like: "I was worried you two won't get along. I shouldn't have worried, I guess." . Yet, it is a conflict that needs to be resolved.
• Near the end of the book, some sort of accident happens. Either horse bucks and the wife falls or something like that. Anyway, the result is that her neck is broken and she is unconscious. I am not certain whether she wakes up to have a last talk with her husband and his friend. What I am certain of, is
• The family physician/ property doctor checks on her. Even if she wakes up or survives whatever happened to her, she will never walk again. The heartbreaking decision is made: the doctor will inject her with lethal dose of morphine and she will perish without pain, instead of suffering the rest of her life as para/quadriplegic, which would be worse than death for a free spirit like her. I am not certain whether she was awake to help them make that decision, but they do just that. I do not know whether it is doctor doing the injecting or her husband or her lover but she dies at the end of book.
• I think that one of character names might be Henry.

Edit: Language is English and it is all happening in US.

That's all I remember. Any help is appreciated.

Answer 1

"The Little Lady of the Big House" by Jack London. It can be found on gutenberg.org

The story concerns a love triangle. The protagonist, Dick Forrest, is a rancher with a poetic streak (his "acorn song" recalls London's play, "The Acorn Planters"). His wife, Paula, is a vivacious, athletic, and sexually self-aware woman, who falls in love with Evan Graham, an old friend of her husband's.

The Little Lady of the Big House (Wikipedia)

The tractor scene:

“Behold,” he said, “the one-man and no-horse farm where the farmer sits on the porch. Please imagine the porch.”

In the center of the field was a stout steel pole, at least twenty feet in height and guyed very low.

From a drum on top of the pole a thin wire cable ran to the extreme edge of the field and was attached to the steering lever of a small gasoline tractor. About the tractor two mechanics fluttered. At command from Dick they cranked the motor and started it on its way.

“This is the porch,” Dick said. “Just imagine we’re all that future farmer sitting in the shade and reading the morning paper while the manless, horseless plowing goes on.”

Alone, unguided, the drum on the head of the pole in the center winding up the cable, the tractor, at the circumference permitted by the cable, turned a single furrow as it described a circle, or, rather, an inward trending spiral about the field.

“No horse, no driver, no plowman, nothing but the farmer to crank the tractor and start it on its way,” Dick exulted, as the uncanny mechanism turned up the brown soil and continued unguided, ever spiraling toward the field’s center. “Plow, harrow, roll, seed, fertilize, cultivate, harvest—­all from the front porch. And where the farmer can buy juice from a power company, all he, or his wife, will have to do is press the button, and he to his newspaper, and she to her pie-crust.”

In the end, she wounds herself mortally with a rifle in what her husband is certain is a suicide.

Doctor Robinson made his examination. When he arose with an air of finality, Dick looked his question. Robinson shook his head.

“Nothing to be done,” he said. “It is a matter of hours, maybe of minutes.” He hesitated, studying Dick’s face for a moment. “I can ease her off if you say the word. She might possibly recover consciousness and suffer for a space.”

...

“How long?” she queried.

“Not long,” came his answer. “You can ease off any time.”

“You mean...?” She glanced aside curiously at the doctor and back to Dick, who nodded.

“It’s only what I should have expected from you, Red Cloud,” she murmured gratefully.

Answer 2

What you are describing can be somewhat related to the Wish Away psychological drama, where however a murder triangle exists. First Nandini Basu kills Siddharth and then Netaji kills Nandini Basu. So there is a sequence: first a young boy gets killed by a middle-aged woman, then this middle-aged woman gets killed by an old man. However, no PC was used in this drama.