Hiero's Journey, which is both a fun book and an avowed influence on Gamma World. I think it sets just the right tone through its difference in geography and nomenclature from the modern world. You know, looking at it, that you're not in Kansas anymore.
Does anyone else know of any great post-apocalyptic maps they'd like to share? There are the Gamma World ones, of course, but I'm thinking primarily of ones from literary settings. I don't recall ever seeing a map of the Horseclans world, for example, but there may very well be one floating around somewhere on the Net. If you know of any especially well-done or evocative post-apocalyptic maps, please mention them in the comments below.