Note, July 25, 2022: I've just edited this to correct a single typo.
Like some other books, this is one that I read only because it was picked as a common read in one of my Goodreads groups. While I'd heard of it before, it had never struck me as something I wanted to read. In some cases, books I read this way proved to be five-star reads. This one didn't impress me to that extent; but I did ultimately like it well enough to give it three stars, and found it thought-provoking on various levels.
It's a somewhat challenging book to review, and even to classify. With regard to the latter point, I finally settled on "science fiction" for its genre, though it's very unlike most American SF from that era. (Nor does it fit into the "lost race" tradition popular on both sides of the Atlantic before and between the World Wars.) But it does have a central speculative element to its plot: the idea of long extension of human life (though not actual immortality, nor anything like it) by purely natural means. This element is squarely in the "soft" SF tradition (more characteristic of the British than the American genre), a literary conceit employed to set up and serve the human social and philosophical questions the author wants to explore. (It isn't based on any serious study of the actual causes of aging, nor on extrapolation from any known technique or effect.)
Apart from two framing sections that filter the main narrative through an effect of, in Washington Irving's term for the technique, "resonance," the premise of the latter is fairly simple. Four people --viewpoint character Conway, a WWI veteran now a British consul; his younger vice-consul Mallinson; a missionary lady; and a rather mysterious American-- being evacuated by air from a local uprising apparently on the northwest frontier of what was then British India, find their plane hijacked by a mystery pilot taking them to an unknown destination far to the East. Any more direct information would reveal plot elements that the author preferred to disclose gradually; and the genuine suspense of reading it with no more knowledge of the plot than is inevitable with normal cultural literacy about a 1933 classic is actually an integral part of the reading experience. For the same reason, I don't recommend reading the cover copy of this edition, nor the Goodreads description; where they aren't inaccurate and misleading, they can be "spoilerish" to a degree.
Basically, however, this is a novel of ideas; the plot exists strictly to serve the author's message(s). These are the messages of a pessimistic, primarily secular humanist British intellectual whose view of the world was deeply shadowed and scarred by the Great War. (The reference to Conway's wartime experience was convincing enough to make me suspect Hilton was himself a veteran. He wasn't, having turned 18 just a couple of months before the Armistice; but he was still part of the rising bourgeois liberal "Lost Generation" that was epochally disillusioned by the scope of the carnage.) He was also clearly hag-ridden by the prospect of a second world war, which he expected to be apocalyptic. (He often gets credit for being brilliantly prescient, but his expectation was more probably the fruit of dogmatic pessimism more than of astute observation of world politics; though the book was published in 1933, I'm guessing it was probably actually written before Hitler became Chancellor. And the actual World War II, though bad enough, was far less apocalyptic than Hilton imagined it would be.) The book is basically a call to preserve the human race's cultural, artistic and philosophical patrimony in the face of its anticipated near total annihilation in the coming war.
Another philosophical undercurrent here is Platonism, which is clearly discernible in the glorification of the supposedly benevolent rule of what are in effect "philosopher kings," morally and intellectually far superior to the docile subject population that they rule for its own good; in the disparaging of emotion and passion as a juvenile enemy of exalted Reason; and in the upholding of "moderation" between two extremes as the all-purpose ideal for human conduct. (Hilton's prep school and Cambridge Univ. education, of course, in his day, would have steeped him in classical thought.) He also has no more real understanding of the religious mindset than a tone deaf person has of music (with the difference that those of us who are tone deaf usually understand that we can't perceive something, whereas that's not an awareness that troubles Hilton). Despite the setting of much of the story in Tibet, actual Eastern philosophy and Tibetan Buddhism doesn't furnish any real contribution to the ideology behind Shangri-La. None of Hilton's basic premises are very similar to mine. But a real value of the novel, for me, was the way it encouraged me to compare and contrast my ideas with his, and to gain insights from that process along the way.
Some reviewers have expressed dissatisfaction with the ending; and, without resorting to spoilers, I can say that I understand why. However, I don't share that dissatisfaction. IMO, the ending was perfectly crafted, both to preserve the element of mystery and ambiguity that's often seen as essential in the speculative fiction tradition, and more importantly to make a human element central to the story arc, rather than reducing it exclusively to a message-driven essay just dressed up as fiction about human beings. That's something the author deserves credit for as a writer.