This is the first book in the Tripods trilogy, (which was then expanded to four books by a prequel volume which, oddly enough, should probably be read last because it gives away too much). Written in 1967, the book is a bit old-fashioned, but in a good way. By that I mean it is a bit slow, but is well and solidly constructed and has all of the virtues and a few of the weaknesses of boy's adventures written during that period.
And this has everything a young reader could want. Alien overlords. A dash for freedom. Resistance and rebellion. Lurking and hiding. Friends and foes. Unlikely alliances and companions. And a vaguely dystopian, post-historical setting with references to "ancient technology" that kids will recognize.
The book is crisply paced and directly written, with enough description and character building and scene setting, but also with headlong action, dollops of suspense, and a sort of headlong rush to book two. That said, this book ends, (SPOILER), with the boys just making it to the rebel stronghold. There's no "cliffhanger" as such, but of course there's lots more story to follow in the later books. (By the way, sometimes this book is referred to as "Book Two", but that's just because it got renumbered after the prequel came out.)
So, a bit of a throwback, but it has held up remarkably well, and is a solid, well made, and entertaining choice for a new sci-fi survival/adventure reader.