So, first of all, the novel spends more time on Ashley and Cecile's youth. We get to learn more about them as a characters and we get more interactions that further their relationship, which I think was a bit lacking in the manga.
I think the biggest thing that manga cut out was Oswell's character. While the manga did touch on it, it simply was not enough. His backstory and his character is far FAR deeper and more twisted than manga ever goes over. I mean there is an entire chapter just dedicated to his point of view and backstory. I'm not gonna lie, I was a little scared of him.
It's not just Oswell, all the characters are deeper and more twisted in the novel version. I think of Ashley's character. In the novel, he is far more obsessive over Cecile. He's also more on edge and more expressive(?) In the manga, Ashley kinda has this cold/bland demeanor. While this is present in the novel, he still feels like he's gonna snap any moment.
Cecile also surprised me. She is way smarter and a bit more twisted than the manga depicted her. Like the whole of chapter 13 manga (the confession) was cute and all, but the novel was a bit more twisted. I mean she threated to cut off an important part of Ashley if he ever went back on his word to love her.
Plot-wise, everything is the same, just with a lot more steps. It's just that the novel takes its time to flesh out the plots and plans.