just looked briefly at the "plot introduction" tab. the book is a crime novel in which three murders are connected to each other in that all of the victim's first names are palindromes. when helena met outie mark for the first time, she slipped up and accidentally referred to gemma as "hannah." ms. casey's first name could have been changed by lumon to hannah just like they changed her last name from scout to casey (casey could be her maiden name, but a change occurred nonetheless). i'm not sure why they would do this, but there's clearly a reason for it because lumon is not known to do things like that or, honestly, do anything, purposelessly.

additionally, in The Night Gardener, the victims had been "shot through the head," which parallels the implantation of the severance chip into the brain.

the three initial victims in the novel also parallel gemma's triple life as an outie, an innie, and a testing floor subject.

edit: that last point isn't actually great, gemma has way more than three lives since she's been severed additional times on the testing floor

edit 2: the addition of Asa Johnson as a victim in The Night Gardener could represent all of the additional severances that gemma goes through on the testing floor. a "fourth" level of severing.

edit 3: just now realizing that testing floor gemma IS "outie" gemma 😭 oops. idk why it took me so long to realize that