Dragons from the Sea by Judson Roberts
Summary copied from the Library Network catalog because my brain has the consistency of milkshake today:
Fifteen-year-old Halfdan's skills as an archer and a blacksmith win him a position on the crew of a Viking ship in 845 A.D., but as war breaks out in western Frankia he becomes a scout, searching for the Frankish army--and taking captive a young woman who is preparing to be a nun.
Though it didn't blatantly suffer for being a sequel, I did feel it would have made more sense if I'd read the first one. Not a bad read, but not an award winner either. NAY.