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Limited nuclear strike on the US around 1989, 1992 (is when the strike occurs in the universe of the book).
Its US v Soviet Union. Limited nuclear engagement is caused by the US deploying an ICBM kill system so advandced it scares the Russians into a "use them now or never be able to use them" scenerio.
The book describes the initial strikes and the days after through the eyes of the main character, then picks up 10 years after. The entirety of the book is third person autobiography, IE the main character has written this book describing his experiences. The book chronicles his travels around what remains of the United States, now several different nations all suffering from endless maladies caused by the war, and discusses all the things he finds and sees.
And yes, California is a communist nation state in the aftermath. With internment labor camps.