<span>The strength of the novel is its slave narrative tone and its ability to demonstrate the pain of being owned by another human being. Many have heard of slavery; few know this story. It's too important to overlook.</span>
This is an example of a simile, a comparison between two things using the words "like" or "as." "Gloomy as the night" is the simile here, since his sadness is being compared to nighttime.
