I’ve read a few Jim Thompson books (all audiobooks) and I would listen to every single one I could get my hands on if more were available. The stories (all weird, alluring, and violent) are attractive and compelling dips into some kind of petty underworld, and usually with a compelling narrator (at least four straight so far for me).
This one dips into some heavy trope territory and as it’s 1955 you have to be willing to put up with some heavy bouts of misogyny and mental health stigmas.
Our narrator is a former boxer newly released from a mental institution who stumbles into a bar and befriends a habitual drinker (a woman whose age he can’t quite pin down because of her life habits) and an ex cop. They mock him mercilessly, given us early insight into his deep sensibility to people using his head injury, mental health, and intelligence against him, even if he doesn’t understand exactly how they’re doing so. He more or less falls into love with the woman, but he runs off, and is picked up by a kindly psychiatrist who understands immediately that he needs help.
So the novel then splits the narrator’s loyalty and attention among these three poles. The plot then comes in as they hatch a plan to kidnap a rich family’s kid (with the obvious idea of pinning it on our narrator) and when that kid ends up having a medical emergency things go awry.
This most reminds me of both Stephen King’s Blaze (and he might credit this one) and Of Mice and Men.
(Photo: https://www.amazon.com/After-Dark-Sweet-Jim-Thompson-ebook/dp/B005IGVSFW/ref=sr_1_1?crid=QFXDCWBCGI0C&keywords=after+dark+my+sweet&qid=1575044572&s=books&sprefix=after+dark+m%2Cstripbooks%2C139&sr=1-1)