Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The mind is its own place, and in itself
Can make a Heaven of Hell, a Hell of Heaven
John Milton, Paradise Lost
What's up with the epigraph?
Geraldine McCaughrean knows nothing kicks off a book quite like quoting Satan. (Well, technically the epigraph was written by John Milton, but it's something Satan says in Paradise Lost.) The quote itself is about subjectivity and how much in life is interpreted according to your state of mind.
This is especially relevant to Sym, the protagonist in The White Darkness, who is an introvert that lives inside her own head as much as she inhabits the external world. Sym sees imagination as a positive force in her life that makes difficult situations seem more bearable. But in some ways, her imagination is also the author of a lot of unnecessary negativity; often Sym is too self-conscious (and self-critical) for her own good. Plus, part of her mind picks up on Victor, Manfred, and Sigurd's shadiness, as evidenced by Titus's comments, but a bigger part of her mind pushes these clues aside in favor of a rosier picture.