John Christopher was a pseudonym of Sam Youd, a buddy of John Wyndham. The setting here is a post-apocalyptic medieval society where young lads get to be princes and knights and shape their destiny and that of their community. It's like an old-fashioned version of Joe Abercrombie's "Shattered Sea" books. "The Prince in Waiting" and its two sequels are good, rather than great books. I have an affection for the author because I like juvenile SF (always have) and he was a pioneer.

"The Tripods", with its 1968 revolutionary vibe, it better.