I do not really get the meaning of stereotypes and representation in literature, how they are depicted in Beloved for example or in Dessa Rose or in any slave narrative? Is there any special book explain thses kind of literary styles?

A stereotype is a fixed, commonly held notion or image of a person or group, based on an oversimplification of some observed or imagined trait of behaviour . Just think of the Simspsons tv show. Matt Groening (shows creator) throws stereotypes into each character. Consider the fat lazy policeman stereotype in chief Wiggum. In Beloved consider Mr. Garner's brother-in-law. Schoolteacher was a cruel and sadistic master, interested in ways to break the wills of his slaves. He conducted a pseudo-scientific study of the slaves, treating them in his study the way a biologist treats lab animals. His nephews held Sethe down and stole her milk while schoolteacher took notes. When it was discovered that Sethe told Mrs. Garner what they had done, schoolteacher had one of his nephews whip Sethe, giving her the distinctive scars on her back. In this case the stereotype might be true. Slavemasters were a nasty bunch, but it is still a stereotype.

But if we describe schoolteacher as a cruel and sadistic master, is not this a representation to his character in the novel?