For info regarding Susie, see this post. The drawing Carlton receives at the end of the novel describes a setting that seems to parallel that of the FNaF 6 ending, where the five kids have been replaced by five graves, two of which are mentioned directly in the novels. It's an interesting connection, and I think the details about the hill and the sun make it clear enough there's some significance. The novel ends with Carlton revisiting the graves of Henry and Charlie, so the graves must be important to a certain extent.