To me, the mood in this book is really quite dark and depressing. I say this because the social conditions that Rusty-James and his family are living in are quite unpleasant.
Rusty-James is living with his father in a poor part of town. His father is an alcoholic who has been like that ever since his wife (Rusty-James's mother) left the family. He has a lot of education but has lost the will to be someone. He cannot understand his kids and sees them as sort of curiosities.
So, to me, the story is very dark. Rusty-James's life seems depressing because he really has no one to trust and no real prospects for improving his life. We get this feeling right from the start when we find out that, five years after the events of the book, Rusty-James is just out of prison and has no aim in life.