In the case of As I Lay Dying, there are more than a dozen narrators, each of whom has their own point of view, and, as post 2 puts it, their own agenda. So clearly it is difficult to establish an objective narrative, which is really the point. In Ethan Frome, there are two different narrative voices, one of which is omniscent third person, and the other is first person. I suppose you might argue that the omniscent narrator is more reliable. In Gatsby, of course, the narrator is Nick, who claims at the beginning of the book that he has learned to reserve judgement on people, though that doesn't always seem to be the case.