Oh dear where does one start?
The obvious inspiration of the Living Dead movies is Richard Matheson's modern day vampire novel
I am Legend. But what Romero does in the course of the series is to expand and elaborate on the basic idea to give us a much more fleshed out narrative about human society in the face of anarchy...not so much in
Night which for all its fame is little more than a well-executed B-movie but
Dawn and
Day of the Dead are both brilliantly realized hypothetical looks at what happens to what we call 'civilization' when all its perceptions of normal are just blown away. First the blind survival instinct kicks in, but after that...trying to come to actual grips with a situation and realizing that every day, hour, minute, second of life has to be lived against this mockingly chaotic background that dares you to remain sane, remain 'civilized', remain 'human' when everything around you is steadily going to pieces.
Apart from the zombies in the background and the occasional breakout into gore (but these are always part of the story and not the gratuitious flesh-mutilation fests of Fulci's films, I hate it when Romero's name is clubbed with Fulci and other gore specialists) the films can also be regarded as intimate and intense character dramas with very fleshed out individuals whose behavior and POV we can definitely understand even when we're not in agreement with. These films are depressing, in the way that some of Orwell's work is depressing, when you really care about the characters, and come to grips with that they are eventually going to be destroyed, that there is no real happy ending for them (**POSSIBLE SPOILER**
Not necessarily in an explicit sense, and Day does pull out a last-minute left field tackle that could almost be regarded as a magic realism twist **END OF SPOILER)
Definitely lots more interesting stuff to share but that'll be when other people participate after seeing the movies.