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King and Emperor is the third (and concluding) novel in the The Hammer and the Cross series. It was written by Harry Harrison and first published in 1996 by Tor Books.
Shef, now "King of the North", sets out to learn the secrets of manned flight and Greek fire while his enemy, the now emperor Bruno, seeks the Holy Grail.
Map of Europe at the beginning of the novel. Shef's Empire of the North is denoted in red.
Harrison, Harry. King and Emperor. Tor Books, 1996.