I love this book; this and Ubik are my favorite PKD books, and I'm nearly a completist.
Dick takes an idea which is becoming commonplace - multiple universes - but pioneers it and give it his own unique spin. Essentially, the people in the book are living a post WWII tragedy, where the bad guys won - and to them, our universe is the good one, they'd much rather be in. From this premise PKD weaves in multiple characters and storylines from when he was essentially at the top of his game. Excellent book.