I would think that since they were paid for and needed to work farms and such that people wouldn't just constantly beat them like you see in movies and television.

This is a very problematic viewpoint that rests upon total human devotion to economic incentive, which is not the case with chattel slavery. It contradicts an enormous amount of primary source material out there, which describes slavery as a dehumanizing, torturous institution in which its victims were frequently abused (physically, mentally, and sexually), denied rights to their traditional culture as well as access to basic education (e.g. literacy skills), and frequently separated from their families to the economic advantage of others.

By 1860 there were ~ 3.9 million slaves, so obviously there are going to be a wide variety of exceptions to the rule. Some slaves did face easier conditions based on the (relative) benevolence of their masters, but there's no denying that on the whole the American slave suffered enormously under a very inhumane institution that overwhelmingly served whites at the expense of blacks.

As for sources, one needn't go beyond the slave narratives that are out there, most notably Frederick Douglass'. As for secondary sources, any recent (post-1950s/1960s) historiography on the matter is going to affirm the validity of these sorts of narratives and synthesize these perspectives into a grand picture of the institution. Check out certain works by Foner, Stampp, E. S. Morgan, or any other qualified historian of the antebellum, Civil War, or Reconstruction periods.