What is the author's style in The Minds of Billy Milligan by Daniel Keyes?

Billy Milligan is written in third person, the only option available to the writer because the story is presented from the perspective of multiple people. The perspective becomes a serious issue at times and some readers may find they are confused about the "people" involved. Again, this seems the only option available to the writer. For example, when Billy's personality named Arthur takes over, there's an English accent and Billy - as Arthur - tends to make very rational, informed decisions and to excel at book work. When Billy is threatened, Ragen takes over. Billy - as Ragen - acts out aggressively, is strong and capable of defending himself. These aspects of Billy's personalities are very specific and are a vital part of the story. It seems the author's two options were to present the story as he did, saying that "Arthur" performed some action or "Regan" took some stand on an issue or to continually saying that Billy did so with some particular personality in control. Either would have been confusing but is seems this was the best possible of the two.