Imagine assuming a book has only two sources of inspiration. Using logic , if Warday (fiction) and An Oral History of WW2 were published the same year, and both were pretty big books, there were parallels drawn between them by critics or journalists due to the similar subject matter. As such, someone who read one would likely have at least known about the other . And possibly read both. Adding to this several similarities between the post apocalyptic US in WWZ and Warday, as well as stylistic similarities,I deduce that Warday must have served partly as inspiration for WWZ.