I read this as a child and vividly remember certain particularly fraught scenes but not the overall plot. I read it again yesterday in one sitting.
This book doesn't age at all -- it just as fresh and readable and compelling as it must have been in 1967 when it was first published. The narrator is young Will Parker, still a child, not yet initiated into adulthood via the mysterious "capping" ceremony, where the child is taken briefly away by towering metal creatures called Tripods and fitted with a metal disc in the skull. This metal cap brings with it a sense of peace and deep desire to serve the Tripods. This divide between the Capped and not-yet-Capped is a deep gulf, interfering even with the closeness of the relationships between parents and children. At the beginning of the story, Will is just beginning to act on youthful urges towards mischief. Disturbed by the change he sees in his closest friend, who has just been Capped, Will takes a chance offered to him by a stranger to escape and make his way from Britain to the continent and into the Alps -- which, a century in the future when this story is set, are now called just the White Mountains. Will faces and overcomes hunger, sickness, and temptation and learns, by fits and starts, how to forge bonds of friendship and trust with two other boys making the journey with him.
I have nothing but praise for John Christopher's world-building, pacing, and tone. We see a future Europe through Will's eyes, limited by his understanding of the technologies and traditions have persevered in this future, but recognizing, as readers, the technologies he doesn't understand. Christianity has survived whatever great battle resulted in the destruction of the great cities and dominance of the Tripods. Will refers in passing to churches and cathedrals as familiar edifices and recognizes the partially destroyed Notre Dame when making his way through the remains of Paris. He also has absorbed at least some Biblical canon, making a reference late in the book to "how David had felt when he saw Goliath topple in the dust in the valley of Elah." It is clear, thus, that the Tripods are unthreatened by religious tradition, especially given that it tends to instill a respect for and acceptance of authority, even if that authority is shrouded in mystery.
In this first installment of the Tripods trilogy, clocks and watches are a recurring theme. Will's first rebellion centers around taking his father's precious watch from his desk and wearing it outside to show off to his friend. We learn from this episode that the not-yet-Capped children are subject to all the same whims and petty jealousies we recognize in ourselves: pride, greed, shame, the desire for status, the need to be recognized and valued. Time is the factor that provides a necessary sense of urgency to the story. Will and his companions are on the cusp of capping age and thus have very little time left to make their way across a continent without suspicion that they are trying to escape.
I hope that this book remains popular among young children because it is an excellent introduction to dystopian science fiction and doesn't talk down to the reader. Christopher's characters speak as twelve-year-old children would, but the inner thoughts of Will are rendered in a full, rich vocabulary that will challenge young readers to expand their own.