What is evidence from the book that shows that the setting helps set the mood?
The most pertinent evidence from Walter Dean Myers' novel Monster regarding the setting's influence on the mood is the opening text.
The best time to cry is at night, when the lights are out and someone is being beaten up and screaming for help. That way even if you sniffle a little they won't hear you. If anybody knows that you are crying, they'll start talking about it and soon it will be your turn to get beat up when the lights go out.
Here, readers see the true reality of life locked up in a cell awaiting trial. Readers can see how fearful Steve Harmon is and it translates for the reader. There is no question that the setting of prison/ jail is frightening (especially for a teen). The opening shows readers just how frightening life in the "slammer" is.
Therefore, there is no question the mood is set for the novel at the very beginning. Steve is very afraid and the description of night in jail translates smoothly.