The Burning Land by Bernard Cornwell

The Burning Land: A Novel (Saxon Tales) - Uhtred is in Northumbria; he has broken his vow to King Alfred and is now fighting with the Danes against the Saxons.  However, he has made a vow to Æthelflaed, the king’s daughter, and that he has to honor.  The Danes are besieging Wessex by burning and retreating individual villages.  After the Danes burn a nunnery, Uhtred captures their leader’s woman.  And the woman leads Uhtred to fight against the Frissans where she claims there is enough treasure for him to raise his own army.