I'm an idiot of course: Philip K. Dick's novels are never about what they seem. The book is brilliant.
First things first: here's an outline of the setting (I hesitate to call it a plot), excerpted from Adam Roberts' excellent review.
"The novel is set in West-Coast America in the 1960s, but a 1960s in which the Axis powers won the Second World War. The old USA has been partitioned, with Nazi-run Eastern and Japanese-run Western portions and a notionally non-aligned buffer-zone along the Rockies.The one- and two-star reviewers on Amazon were disappointed that the novel isn't the American-noir they were expecting. The Wikipedia article gets it better:
"The main characters are a number of San Francisco Americans getting on with their lives in their various ways. The Japanese overlords are totalitarian but honourable, and many of them have a penchant for collecting old US-memorabilia -- civil war pistols, 1920s comic-books and the like, a market a number of Americans are happy to supply. Meanwhile the Germans have landed astronauts on Mars, drained the Mediterranean for farmland, and have almost entirely liquidated the black African population in an extension of the 'final solution'.
"Dick's use of detail to sketch out his alternate reality is well-nigh flawless, a model for others to copy: not too egregious, but always suggestive and thought-provoking. But the masterstroke of the book is the character Hawthorne Abendsen, the author of the pulp bestseller The Grasshopper Lies Heavy. Living in the castle of the book's title in the Rockies, Abendsen has consulted the ancient Chinese Book of Changes, the I Ching, and used his readings of it to write a sort of Science Fiction potboiler, an alternate-reality tale in which America and the Allies win the Second World War. His book has become a popular success, so much so that the Nazi high command want him assassinated."
"The interpretation and confusion of true and false realities is the principal theme of The Man in the High Castle; it is explored several ways:The I Ching - so central to the novel - is a Taoist work, a tool in seeking the wisest, most harmonious course ("the Way") within life's eternal struggle between opposites (the dark and the light, yin-yang). Dick has the I Ching as the real 'author' of the novel-within-a-novel The Grasshopper Lies Heavy, itself a representation of "The Man in the High Castle" which we know Dick wrote by consulting the Book of Changes.
- Robert Childan grasps that most of his antiques are counterfeit, thus, becomes paranoid that his entire stock might be counterfeit; a theme common to Dick's writing (cf. Do Androids Dream of Electric Sheep?), wherein the counterfeit is better than the original, because it is functionally real, e.g. the .44 caliber Colt Army Model 1860 revolver indistinguishable by anyone but an expert armorer, as Tagomi's shoot-out demonstrates.
- Several characters are secret agents traveling under assumed persona and pretenses; the gentile "Frank Frink" is the counterfeit persona of the Jew "Frank Fink".
- The Grasshopper Lies Heavy, the book-within-a-book, postulates an alternative universe where the Axis lost World War II to the Allies, albeit with an alternative sequence of events. It is an alternative history analogue of The Man in the High Castle. The interpenetration of two false realities illustrates that the idea of a false and a true reality is inaccurate, because there exist more than two realities.
- Novelist Hawthorne Abendsen, the eponymous Man in the High Castle, lives in a house after having lived in a castle (fortified house) that was more prison than home, yet, for the sake of perception (false reality) he perpetuates the myth of his fortified isolation.
- At the end of the novel, Hawthorne Abendsen and Juliana Frink consult the I Ching—it tells them they are living in an immaterial (false) world.
If you like this kind of self-referentiality, you'll find this novel awesome!
My take on "The Man in the High Castle" is that Dick is trying to work out how different life-philosophies deal with good and evil. The Nazis, as depicted, are stomach-churningly nasty, sadistic and genocidal. Westerners such as the ethical Abwehr agent form an active, highly dangerous resistance; the Buddhist character Tagomi struggles to reconcile non-violence with the need to confront really bad people while the artisan characters of Taoist folklore (the Jew Frank Frink and his estranged wife Juliana) seem to act harmoniously in accordance with their true natures - in the moment as Taoism suggests.