I've just reread this - it was one of my favourites as a kid (moreso than either of the sequels) and I still really enjoy it. It's solidly science fiction, with the Tripod invaders enforcing a new world order through mind control... but the order is calm and attractive, and the people under it are happy, for the most part. They just don't retain freedom of thought after having all their doubt and scepticism stripped from them. Without the ability to doubt, it's stagnation all the way...