Criminal psychologist Seonkyeong receives a surprising call one day. Yi Byeongdo, a serial killer whose gruesome murders shook the world, is willing to break his silence and speak exclusively to her. Intrigued, Seonkyeong agrees to interview him.

That same day, Seonkyeong's husband brings home Hayeong, his eleven-year-old daughter from a previous marriage. Hayeong has been living with her grandparents since her mother's mysterious death, and now her grandparents have been killed in a house fire.

Seonkyeong is determined to make the child feel at home, but to her husband's frustration she finds it difficult to connect with the child who says little and acts strangely. It's not long before Seonkyeong notices similarities between her stepdaughter and Yi Byeongdo: they both look at her with cold eyes, each possessing an unnerving calmness that hides a fierce, cruel temper.

As Seonkyeong begins to question details of Hayeong's past—and as her relationship with Yi Byeongdo grows more complex—she begins to wonder what her young stepdaughter is truly capable of.

Sleep Well, Mommy has a story by Mi-ae Seo. English translation is done by Yewon Jun. Published by Ecco (September 15, 2026).

Content Warning: animal death, animal torture, murder

Lauren Orsini
Rating:

Are serial killers born or made? Sleep Well, Mommy is a thrilling book that tackles this very question. Through the eyes of Seonkyeong, a criminal psychology professor with huge Final Girl energy, we explore parallels between two characters: the charismatic convicted serial killer Yi Byeongdo and the traumatized little girl Hayeong who may be more sinister than first impressions suggest. Both of these characters have their own reasons for seeing Seonkyeong as their mother figure. But in Sleep Well, Mommy, a mother is the most dangerous thing a person can be.

Written by Mi-ae Seo, a bestselling author and TV writer in the thriller genre, this story strikes me as more literature than light novel. But while its actual content (with trigger warnings for animal torture, animal murder, and of course human murder) can be on the heavy side, the prose itself is quick-paced—the kind of book I found it difficult not to finish in one sitting. Seonkyeong is thrust into two simultaneous situations outside of her control that lead her to become both a displaced 11-year-old's caregiver and the only psychologist an infamous murderer would agree to speak to. As Yi Byeongdo spills lurid confessions of his abusive mother and how her actions prompted him to start killing women who reminded him of Mommy Dearest, Hayeong begins her first forays into animal torture (at the same time that the hapless Seonkyeong unknowingly buys her a new puppy! I'll save you some heartache now and reveal that the dog dies). For a professor who instructed her class on the [sic] “McDonald's Triangle” at the beginning of the book (the MacDonald Triad, the three childhood traits that all serial killers are believed to share), she is comically unequipped to deal with Hayeong's rapidly escalating problems—or the growing danger she doesn't realize she's in.

Other than the trigger warnings that will be instant barriers to entry for some readers, this is a gripping page-turner that had me yelling at my e-reader for Seonkyeong to get a clue already! Beginning about 70% into the book, things spiral out of control so quickly that I had to finish it in one go. It occasionally tips from frightening to unintentionally(?) comedic: when Yi Byeongdo corners Seonkyeong, saying he isn't sure whether she reminds him more of the Wire Mother or the Cloth Mother in his life, she replies earnestly, “I'm neither of them. There are other people in the world besides [your mothers!]” In this twisted Electra-complex book, mothers are perceived as all-powerful deities while in reality they're as vulnerable as victims can be.