Saxon: The Book Of Dreams by Tim Severin

Frankia 780AD: Sigwulf, a minor Saxon prince, is saved from execution after his family is slaughtered by the ruthless King Offa of Mercia. Thanks to his Devil's Mark – his eyes of different colours – Sigwulf is exiled to the Frankish court of King Carolus, the future Charlemagne. There Sigwulf survives on his wits while at the same time trying to come to terms with disturbingly prophetic dreams.

He gains the friendship of some – Count Hroudland, Carolus' powerful and ambitious nephew but – mysteriously – several attempts are made on Sigwulf's life. When he obtains a Book of Dreams by chance, a rare text giving understanding to their meaning, he attracts the attention of Carolus himself. But the Book proves to be a slippery guide in a world of treachery and double dealing. Carolus sends Sigwulf and his slave Osric into Spain to spy on the Saracens ahead of a planned Frankish invasion. There, Sigwulf becomes caught between loyalties; either he honours his debt to new friends among the Saracens, or he serves his patron Count Hroudland in his quest for glory, gold and even the Grail itself.

One after another Sigwulf's predictions come true, but often not as expected, and he finds himself swept forward into a final great battle that reveals who his enemies are…