There are 3 books in the Hammer and the Cross series. The newest book is King and Emperor which came out in 1996.

1
The Hammer and the Cross

Book cover of The Hammer and the Cross
In an alternate history set in A.D. 865, Shef, son of a Norse raider and an English lady, tries to carve out a kingdom of his own in England, while Christian kings and Viking worshippers of Asgard battle for the country's dominion.
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2
One King's Way

One King's Way
An alternate version of the Dark Ages is set in a Viking-conquered England, where a mighty new ruler plans to build an unstoppable navy of catapult-bearing warships while preventing his many enemies from rising against him.

3
King and Emperor

King and Emperor
In the third book in the The Hammer and the Cross series, Shef, the visionary and warrior, is forced to defend his Northern kingdom against the might of a reborn Roman Empire.