How is Grandma's shed important to the plot in the novel, Long Division?

Grandma's shed is located on Grandma's Melahatchie property in Book One. City knows that the shed is off limits, and is careful never to enter. However, after Grandma comes home one night with bloody hands, and tells City not to enter the shed, his curiosity piques. He then steals Grandma's keys, and sneaks inside. There he finds Sooo Sad lying on the floor chained, gagged, and beaten up. Over the course of Book One, City repeatedly returns to the shed in order to visit, talk and read to Sooo Sad.