Actually, he's quite the opposite. In the books, Sonny is described as being a kind and loving man to both his wife and children. Although he is most certainly a bad Don (something that his own father, even after Sonny's death, couldn't deny) he is a wonderful father and husband. His infamous temper is never taken out on his children or wife; Sonny is described as never being able to bring himself to hurt something helpless, especially women and children. As for his infidelity with Lucy Mancini, there's an explanation for that in the book as well. In a highly unusual turn of events, Sonny's wife, Sandra, is well aware that Sonny sleeps with other women and actually prefers it that way. It is described in the book that Sonny Corleone has an abnormally large penis, so large that it actually gives his wife stomachaches. Lucy Mancini, who it turns out has a congenitally loose vagina, is able to deal with the size of Sonny's penis and thus they start an affair. Their affair, however, is purely sexual: Lucy doesn't love Sonny and barely knows him outside of their affair, and Sonny is purely in love with only his wife. This is slightly alluded to in the film. During the wedding at the beginning, Sonny's wife is talking with other women at the table and she holds her hands up in a "this big" pose and gradually holds her hands out wider and wider, much to the amazement of the other women. She turns to Sonny, but sees that he's gone and a slight look of hurt and/or despair crosses her face. The scene then cuts to Sonny having sex with Lucy. Lucy has a much larger role in the novel, mostly due to an entire subplot, including several main characters not found in the film, including a plastic surgeon, who "repairs" her large vagina, his and Lucy's back-stories, plus their eventual involvement after her affair with Sonny ends (due to his death). This subplot has little to do with the main story of Vito and Michael, and so it was easily excised for the screen version.