If I am correct with my dates, I believe this book was published shortly after the success of Marion Zimmer Bradley's 'The Mists of Avalon.' (1983) If so, I think it is no coincidence. The book has the same sort of formula - the focus on the woman who is the lover of the famous warrior (in this case, the historical eleventh-century Viking warrior Thorkell the Tall, who later became Earl of East Anglia), and more than a hint of neo-paganism.
Our protagonist, Branwen, is the sister of the Earl of Wessex - the enemy of the Danish Vikings, including Thorkell, whom she falls in love with. She is also, although she doesn't at first know it, the last hereditary priestess of the ancient Goddess, and has fey blood that allows her to speak with animals and summon shape-changing illusions.
However, the historical and romance elements of the book are much stronger than the fantasy elements.
Overall, a decent historical fantasy, although at a few points it seems that the plot fails to deliver the facts that the author wants to impart, and she breaks 'voice' to insert some factual historical details, which is slightly distracting. Two sequels to this book were also published, but 'Unkindness of Ravens' definitely works as a stand-alone novel.
Historical fantasy - well written