I came to this book with curiosity, having read the original trilogy when I was at school and having recently had my interest in the series rekindled by featuring the BBC adaptation in my podcast Very British Futures and subsequently listening to Tripodcast.
Overall I have to say it is a missed opportunity, considering the possibilities of a story about Earth being conquered by enigmatic aliens. A lot of the action takes place off the page, and often isn't even conveyed by news reports. So the horrific battles between people and humans who have been converted into willing slaves of the Tripods are often reported in a sentence or two. There's little sense of cataclysm or impending threat. Apart from the first Tripod encounter, which is described in a reasonable amount of detail, the other appearances of the Tripods are quite dryly written and any action recorded in the briefest manner.
The early "Invasion of the Bodysnatchers" element, with the Tripods programming people via a television program called The Trippy Show, should build into claustrophobia but instead feels more like a mild inconvenience. Only one member of the hero's family is affected and since she's had barely any personality prior to being taken over, it doesn't have much impact. Even the programmed humans are still pretty polite and reasonable, giving our heroes plenty of margin to escape from them several times.
As with the later trilogy, the story is told from the point of view of a young teenage boy, in this case called Lawrence. Although never stated specifically, Lawrence seems to be on the low end of the autism spectrum. He himself says he has difficulty forming attachments and envies how his Pa can talk easily with Lawrence's best friend, Alan, but not with him. In the early part of the book Lawrence mostly lists all the things he doesn't like about other people, which makes him dreary company. He made me think of Adrian Mole without the unintentional humour. He does go on a small arc towards being a more positive, richer person but his deadpan narration goes a long way to dampening the excitement.
There are good moments, such as the capped airplane pilot who announces he is going to become a farmer shortly because "people are happier staying where they are". A surprising moment of grisliness when the a farmer abducted by the first Tripod is reported to have been discovered partly dissected inside the wrecked machine. Some of the conjecturing about the aliens within the Tripods is interesting.
Overall, though "When the Tripods Came" is definitely best read at the end of trilogy, rather than as a prequel, since it doesn't do the fun adventure qualities of series justice. Whilst I am reading this book as an adult, I fear that many child readers will find the prose too flat and sombre.
In his preface, John Christopher complains about the BBC adaptation and in particular a review on the BBC show "Did You See...?" where author Brian Aldiss' criticised a particular aspect of the Tripods' design. It's hard to shake the feeling that the drive behind this book was more to address that note than because he had a burning desire to revisit his old creations.