Walnut Milk (くるみのみるく) is the best NTR novel on Syosetu Nocturne. An edited version will be published on Japanese Kindle.
Story is about the protagonist and his wife, Kurumi (synonymous with "walnut" or "くるみ"), who is an extremely kind-hearted school teacher. They have a happy life, though their night life could be better.
One day, when helping his wife's computer issue, he finds a hidden folder. This unfolds a series of events where after telling Kurumi it was a virus that he removed, he later poses as a hacker and virtually blackmails her to learn more about her past.
The focal point is the wife's involvement with a father-son duo who were involved in a horrific accident. The father is now a disabled ex-painter, and the son has burn scars and mental trauma.
Lascivous paintings are uncovered. The chaste wife becomes increasingly erotic. The couple's love life heats up. The past is slowly exposed.
There are 99 chapters (88 in main story and an 11 episode side), where each is important to read. It takes several hours to read in full but MTL (I used Google translate) works fairly well.
It has a happy ending from the author's PoV, but some readers find it contentious; it is genuinely intended to be happy, however, even though certain intentionally ambiguous parts result in motivations and parts of the plot to be deliberated. A series of hints are included throughout, and the author masterfully raises and lowers tension where appopriate.
I 100% recommend this web novel to anyone in this genre. I encountered it years ago but hadn't read it then, but happily revisited it recently. If anyone can find anything better then let me know.
This series is not perfect, but it is extremely gripping. I love it. At some future point, not anytime soon, I'll re-read it since various parts of the novel will appear differently, knowing in advance the twists and turns.
In summary, 5/5.