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I'm apparently in a bit off a minority, in that I found this trilogy only improved with each installment.
I've heard the complaints, chiefly, that the constantly shifting set of characters with similar names makes the plot hard to follow and the constantly shifting setting makes the individual plots too short and unsatisfying, and that changing the focus away from the macroscopic and microscopic universe to the temporal Earth is too great.
But, I politely disagree. The bite-sized plots are each masterfully realized, helping to reduce the difficulties a reader might have telling them apart and throwing into contrast the ways in which each generation is different from others. And while I understand why many budding scientists might be unhappy that these books they used to like for talking about good old science and math are now going on about boring old history and sociology, at the end of the day, history and sociology have always been series themes, and introducing time-travel so extensively is a logical extension.
Furthermore, since we've been with several of these characters for several books now, we finally get to see them complete their development. Charles Wallace, years after he annoyed you with how everyone wouldn't shut up about how great he was in A Wrinkle in Time, finally finishes his arc by growing beyond his arrogant self-importance. Meg, finally grown out of her awkward teenage years and into womanhood, is a calmer and more-perceptive person, carrying her first child and providing an important link between Charles's adventures in space and time and the temporally-locked cast awaiting news of the nuclear end at the Murry house. It's a shame we don't get to see Calvin at all, but he'd basically completed his development last book, and in this one we get to learn so much more about the environment that created him, delving for the first time into his mother, Mrs. O'Keefe, and what made her who she is today.
And the history sections convey important themes. They stress that even though violence has been common, expected, normal throughout human history, it is still wrong, and it is still something that human beings can and should rise above. They look over the universal, recurring bigotry that dogs seemingly every time and place, and condemn it, reinforce the idea of universal brotherhood. And while some take issue with the way the story seemingly paints some family lines as "tainted," they often ignore in the process that Calvin himself is a product of such a line, proving that rising above one's environment can be done... and also that the whole point is to stress the way in which mental pollution and toxic ideas can breed within families over time.
The mystery of what, exactly, is being done to prevent the end of the world, and the wonderfully ambiguous way we, the audience, are never sure what's actually going different in the various branch points make for an excellent palate cleanser to decades of over-detailed, over-cooked time-travel plots. And the poetry that girds the whole book together is catchy and impactful.
The words "timeless classic" get thrown around a lot, often onto books that actually age very poorly in terms of writing, if not theme or story. But A Swiftly Tilting Planet truly is timeless, not even too bogged down by its Cold War time period. My only recommendation is that it works best as the capstone to the trilogy, rather than as a standalone, but even then, there's no Continuity Lockout, and I only mean that it lets all these characters finally finish their journey.
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