Reality Subtext: Philip K. Dick actually did consult the I Ching while coming up with ideas for the book, much as Hawthorne does when writing his. This may imply he is something of an Author Avatar.
Science Marches On: The mentions of Nazi colonies on Mars and Venus by 1962. Both the ease of getting to those planets, and the habitability of the latter, were overestimated in The Sixties.
What Could Have Been: Dick toyed around with several ideas for a sequel, but he couldn't bring himself to do more research on Nazi Germany, which he found profoundly depressing and soul-draining enough for one novel. Among them were:
The first draft of The Ganymede Takeover.
A story called "Ring Of Fire" where the Nazis discover our world and steal nuclear weapons while a hybrid Japanese-American culture emerges.
An early version of Radio Free Albemuth where the VALIS satellite was communicating Hawthorne Abendsen's next book to him and keeping the Gestapo away.