I didn't find out about this series until I was an adult, but I would have loved it as a kid. I love the idea of a sort of post apocalyptic future in which alien overlords have regressed humans back to a medieval society. The descriptions of the empty hulls of big cities, especially the journey across Paris, capture the imagination while giving plenty of scope to imagine one's own scenarios. I just love Beanpole's fascination with the remains of modern technology that we come across. There's a sci fi element, of course, in the cappings and the mysterious tripods, but the book is really more about human nature and whether we truly want to have free will. I also was reminded of the idea in Diana Peterfreund's For Darkness Shows the Stars that human beings want to innovate, and even if our current modern world were suppressed, we'd start figuring out how to do these things over again. I think this concept is particularly gripping for kids who are just learning about how technology works for the first time. 
 Although I think this book was probably YA when it was written, it seems much more of a middle grade read now. My ten year old devoured it, and I appreciated that there was not a hint of bad language or sensuality. Even Will's sortof romance with the daughter of the French aristocrat is subtle and poignant without making the characters do and say things that are unnaturally mature for their age (this is a complaint I have with the romantic interest in the Gregor the Overlander books--I know I'm an old 34 year old fuddy duddy, but romantic tension between tweens and young teens is totally unbelievable and uninteresting). With that said, I'm not sure my girls will be as into this series as my son, because besides Eloise, there is a total dearth of female characters. Literally, none. I guess this is what people mean when they talk about old sci fi/fantasy being so male-centric. I'm not offended by this, honestly, because I think the pendulum has swung so far in the other direction that I have trouble finding contemporary fantasy books for my son that have engaging male characters who don't just serve as points of a love triangle with the super cool female protagonist.