Eh...it was okay, but not recommended unless you're a hardcore Seton fan.
-spoilers ahead-
The story begins with Merewyn & Rumon's time at the court of Edgar & his son Edward (roughly 970-80 AD). The secret of Merewyn's non-royal birth is a deathbed vow that Rumon carries only by accident; he's a petulant, sanctimonious whinger who thinks entirely too much of his worth to God's creation. Merewyn was more likable, though something of a dim bulb, so she doesn't have much personality besides providing a bookmark for various historical settings. Eventually she's captured by a shipload of Vikings -- one of which turns out to be her blood-father -- and is forced to settle for life on Iceland. Despite her misgivings, she grows fond of her father & marries his foster son, Sigurd, then moves with them to Erik the Red's colony in Greenland.
Several years later, Rumon manages to find her...but now she's in love with Sigurd & content with her lot, so Rumon sulks away to become a monk. Fast forward sixteen more years & widowed Merewyn is leaving Greenland with her kids. She ends up married to a rich English dude & we're treated to several episodic snapshots of Ethelred's reign, at which point Rumon-the-Monk drops back into Merewyn's life, somehow convincing her that his opinion matters, & she spends a few pages confessing that she's a Viking bastard. The end.
...Yawn.
At first I was intrigued by the story, despite a meh pair of leads. But the second half blows. We don't see any of Merewyn's acclimation to Viking life, the developing relationship with her father & Sigurd, or even the process of settlement on Greenland. FAIL. The episodic nature of the Viking section really soured my response to the entire story -- which sucks, because I enjoyed the Viking parts that were included. But in the aftermath of important character & plot developments that were so heavily glossed, I wasn't in the mood for Merewyn's meandering around England to highlight a parade of infodumps re: Ethelred's reign. The last 100 pages was a definite slog that inspired zero interest.
Anyway, the first section managed to include some interesting bits about the Dark Ages & (peripherally) the roots of the Greenland settlement. I've read another novel about said doomed experiment -- The Thrall's Tale -- and really enjoyed it. But The Thrall's Tale succeeded where Avalon failed -- that is, crafting a brutal, visceral story that centers on a small cast of characters as they survive an equally brutal, visceral environment. Avalon's scope was too broad, too dry, & too much I AM HISTORY(tm) for genuine involvement. I much prefer books that don't buff away the grittiness to flaunt a sheen of research & beauteous language. Sound grammar & background is good, yes...but vivid characters & involving plot will trump the occasional linguistic wobble or bending-of-facts any day of the week.