The Summer of the Danes
by Ellis Peters. This is the eighteenth installment of the Brother Cadfael mystery series.
This story is a little unusual because there is no crime in Shrewsbury to solve. Instead, Brother Cadfael accompanies his friend Brother Mark (a former monk of the abbey who went off to study for the priesthood) on a diplomatic mission to Wales. They find themselves in the middle of a family quarrel teetering on the brink of a civil war in which one side hires mercenary Danes (Vikings) from Ireland. Brother Cadfael is captured by the Danes and he and Brother Mark help negotiate the peace.
This was a really entertaining story with a satisfying ending.