The zombie fighting games (called "Zilight") are separated by time, sometimes centuries, and/or continents. The original is set in present-day England, one of the games is in present-day Seattle, and there's both a medieval adventure and more recently one set in 1920s Egypt. The one they're working on now is "Zilight Wild West".
There are two main series, each entry being stand alone, although the sci-fi games, under the heading of "Starship Scavengers", can be linked together. Hart had also written "Neo-City" set in the same universe, but on a core world as opposed to most of the "Scavengers" games being out in the fringes of space.