In this alternate world novel Germany and Japan were the victors in World War II, and they have split the conquered United States between them. The principal protagonist, Robert Childan, runs a memorabilia store and sells Americana to the Japanese overlords.
The novel also features a sympathetic Japanese businessman, a judo instructess who goes in search of the reclusive Hawthorne Abendsen, the "man in the high castle," and Abendsen himself — who has written a SF novel in which Germany and Japan lost World War II.
The novel questions our whole notion of reality, and is probably Dick's finest work.
Cover scan of 1st edition via www.isfdb.org