The Last Kingdom by Bernard Cornwell

The Last Kingdom (The Saxon Chronicles Series #1) - Uhtred is ten.  The Danes are invading England and Uhtred goes with his father to fight the Danes.  The English lose and Uhtred is taken prisoner by the Danes.  Uhtred spends the next several years as part of Ragnar’s household.  As he grows to love Ragnar as a father, Ragnar treats him like a son.  Uhtred fights with the Danes until an enemy of Ragnar kills everyone in Ragnar’s company except Uhtred and a girl that Uhtred was with.  Uhtred is then pressed into King Alfred’s service to fight the Danes.