Uther & Igraine (Historical Novel). Warwick Deeping

Uther & Igraine is a romantic tale of the forbidden love between Uther Pendragon, legendary king of Britain, and Lady Igraine, wife of his enemy Gorlois, Duke of Cornwall. After many misfortunes Lady Igraine eventually gets tricked into marrying Gorlois, but their marriage is unhappy. The death of Aurelius Ambrosius made his brother Uther the new king and, under the threat of new battles, Uther called for gathering of knights, many of which answered the call. Duke Gorlois had come from Cornwall with many knights, bringing his wife along. Upon meeting Igraine, Uther desperately falls in love with her, but hesitates to act on it, unwilling to cause harm to her reputation and lose an ally. Lady Igraine, who is miserable in her marriage, returns the affection, but the dilemma they are facing is not only moral, and they'll have to overcome many tribulations.