THE EDGE OF LIGHT By Joan Wolf NAL Books. 371 pp. $18.95

JOAN WOLF, whose favorite playground is the Dark Ages, has once again plunged into a tumultuous time in 8th-century England when betrayals and assassinations were an everyday occurrence.

Loyalties were ever changing as power shifted hands constantly: Whoever carried the biggest stick attracted the largest following. This was nowhere more noticeable than among members of noble families, where cunning and deceit were commonplace as junior sons tried to shoulder aside older sons in order to climb up the family tree, and heirs plotted to speed their inheritances, especially in cases where aged fathers were involved.

Previously Joan Wolf whetted our appetites with a masterful retelling of the Arthurian legend, The Road to Avalon. This was followed by Born of the Sun, a clear, precise depiction of life among the Saxons and Celts. Now the stage is set for the conclusion of this exciting trilogy.

Ethelwulf, the 53-year-old king of Wessex, has just returned from a pilgrimage to Rome with a new bride, 16-year-old Judith, princess of France. In his absence he had left his two oldest sons, Athelstan and Ethelbald, joint custodians of the kingdom. He is met in his hall with the shattering news that Athelstan is dead and that Ethelbald, in open rebellion, has taken the whole kingdom into his hands.

In this turbulent period of English history, the country was made up of several kingdoms constantly warring among themselves. Meanwhile, the Danish hordes were plundering the coasts and indulging in all manner of torture and blood sacrifices.

Out of the chaos steps Alfred, the fifth son of Ethelwulf. As the family feud progresses, he finds himself declared heir to the entire kingdom, second only to his favorite brother Ethelred now king of Wessex after Ethelbald's death.

However, now the Danes become a factor in the kingdom's fortunes. In previous years, they had been content to simply swoop down in their galleys on coastal towns, to kill and loot and then flee back to their own lands. But, later, bolder and greedier, with land become scarce in their own country, a Danish army of 7,000 invaded and settled in England.

The Danes devastated Northumbria, killed one of its kings in battle and captured another. They subjected the latter to the abominable practice of "blood eagle," in which, "while {the victim} still . . . was living, they cut out his ribs and his lungs and spread them like eagle's wings in an offering to their god, Odin."

Wolf's descriptions of a long-ago, mostly rural England are colorful, her portrait of what life was like in those days convincing.

As in previous Joan Wolf novels, there is a romantic subplot. This one features the coming together of Alfred, the 18-year-old prince of Wessex, and Elswyth, a 14-year-old Mercian noblewoman with raven tresses, deepest blue eyes, high spirits, and a stubborn disposition, who is far happier among horses and dogs than ladies of the court and needlework. She implores Alfred to marry her and save her from being forced into marriage with an older Mercian nobleman whom she hates and fears. Alfred has sworn he would never marry because of a strange malady composed of violent headaches and upset stomach which doctors can find no cure for, but he feels drawn to Elswyth and agrees.

Throughout this fairly lengthy novel, Wolf holds the reader's interest. There are no plodding passages. The picture she paints of Alfred the Great is especially winning. She shows a fearless leader but above all a very religious king and one who would be instrumental in making education possible to his people since he himself realized the importance of the written word. To Wolf's credit, the reader finishes her novel with an understanding of why Alfred is the only English king to have been honored with the epithet "the Great." Brian Jacomb is an avid reader of historical fiction.