Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person (Omniscient)
We spend most of Alas, Babylon sitting safely inside Randy Bragg's head. Although there isn't a ton of insight into Randy's emotional experience, we're shown every step in his thought processes as he adapts to the nuclear apocalypse. That's very much in line with the overall purpose of the book: it wants to show us the big picture, societal impact of a nuclear war, not necessarily its impact on an individual's emotions.
And the narrator will occasionally leave Randy's head to gain a different perspective.
Sometimes this is only for a cute scene or two, like when Peyton catches several huge fish without her mom's permission. At other times, however, this is used to give us context. We're thinking of the sections on military maneuvers in the Med, or Mark's activity inside The Hole. In both instances, the narrator gives us insight into the disaster that's otherwise unknowable to its isolated surviving characters.