'Castle' is an alternate history novel that takes place some fifteen years after World War II. This history diverged in the 30's as Roosevelt was assassinated and his terms were replaced by ineffective, weak and isolationist Presidents. That leads the US to be totally unwilling to help it's European allies and unprepared for Japanese attack. As a result the Axis won the war and split the US down the middle with Japan controlling the west coast and the Germans controlling the area east of the Mississippi.
The story follows half a dozen people mostly in the San Francisco area. It shows the big cultural differences between the German and Japanese conquerors and how the vanquished are reacting. The I Ching has become a part of everyday life and is widely used as a daily oracle. Also interesting is a popular novel in the book written as an alt-history in which the Allies win.
This is a very good book. It shows a highly textured and complete world. I'd call it a must for WWII buffs, especially students of the German political set up of the period. The only drawback is that it ends without real resolution.
Of interest to me is that Dick wrote the novel using the I Ching as his guide. He cast for guidance at each plot divergence and essentially let the I Ching do the writing for him. That pretty much guarantees an unpredictable plotline.
I'm also not sure that this really belongs under the 'science fiction' label. Interestingly, Dick makes an argument in the book itself that alternate histories belong there. If you read the description as 'speculative fiction' then it fits rather neatly. In the end it was voted on by people at the science fiction convention and it obviously cleared the bar in their estimation. I just don't think I would have gone that way.