A book about the conversion of the Scandinavian peoples.
Starts with an overview of the original situation and goes through such things as the conversion of Viking settlers in Great Britain. Tended to be very fast in regions where they were few -- the Viking rulers of York were coining coins with the inscription "St. Edmund the King" a generation after the Vikings had killed him, despite no missionaries being sent -- and slower where they were massive in number, especially in the Orkneys where they may have engaged in active ethnic cleansing.
Ireland and where they settled and how they integrated and became Christian as well as Gaelic speaking and intermarrying with the Irish. (Later Irish chroniclers tend to call them pagan to make a simple fight over the independence of a trading city look more noble for a king.)
Also in Scandinavia itself, where records tended to arrive with Christianity and writing. The support of kings -- and women. Then Russia, when the Rus went there, and America, Iceland, and Greenland.