From what I take from this, I think the point of the assignment is to demonstrate that you understand the difference between tone and mood. According to this:
Tone vs. Mood tone is the attitude of the author towards the writing while
mood is how you feel based on the atmosphere of the work.
So maybe for your illustrations, think about that difference in tone and mood. For tone, make an illustration of how you think the author is thinking towards his (it's he right?) work. Is he sarcastic or is he serious? For mood, make an illustration of the kind of feeling you are getting from the atmosphere. Do you feel that the work is mysterious? Sorrowful? Suspenseful?
Basically, the illustrations are not necessarily about what is going on in the book but based on the author's attitude and how you feel towards the book, which are very different things. I'm not saying this is what you should do for this book, but here's an example.
Maybe the author is actually pessimistic about the outcome of his characters, so in this case an illustration of one of those things that keep the characters from returning to their bodies? Maybe because of this atmosphere you feel that this is mysterious so perhaps an illustration of someone in search for answers like a detective would show the mood?
I hope this helps with your assignment. I'm not familiar with the book and it would be easier to determine tone and mood by actually reading it, but hopefully this helps spark some ideas for images.