In to note here that Yout, as John Christopher, also penned the highly-influential post-apocalyptic YA series, THE TRIPODS, depicting humanity reduced to serfdom and agronomy after the conquest of Earth by aliens known as "The Masters".

en.wikipedia.org/wiki/T…

(I'm convinced that L. Ron Hubbard ripped this series off and used the milieu and some other details for his own post-apoc, BATTLEFIELD EARTH.)