Crown of a King: Book Three of the Carolingian Chronicles

It's 745. The kingdom is split between the two eldest sons of Charles Martel. Carloman, the oldest, has the support of the king and the church, while Pippin refuses to bend the knee. Civil war is imminent. Making matters worse, Pippin's relationship with Bertrada, the great love of his life, is no less complicated, forcing him to reevaluate the moral implications of his rule. Just as he believes he's found a way forward, Carloman's campaign in Alemannia upends the peace and puts the future of Pippin's relationship - and the kingdom - in jeopardy.