John Steinbecks' novel, Of Mice and Men, has an undertone of racial prejudice based upon the treatment of the ranch hand Crooks. Although the word "nigger" is no longer accepted as being politically correct, the novel is splattered with the term (when other characters are either speaking to Crooks (the only African American character in the novel) or referring to him outside of his presence). During Steinbeck's time, the use of this, now, derogatory word was typical and accepted.
As for the importance of racial identity in the novel, one can assume that a person needs to be white in order to be looked at with respect and, therefore, accepted as a part of the whole. Crooks is not only verbally barraged at times, he is isolated from the rest of the ranch hands (being given a living area, the harness room, away from the other men).
At no time in the novel, other than the giving of the whiskey to all of the ranch hands, is Crooks ever included in the goings on of the other men. He is constantly ostracized by all of the men (outside of Lennie who fails to know any better than to converse with Crooks on a personal level).