This is an unusual book in that it is ¾ history (herstory?) and ¼ sensible imagining. The imagining comes into play when O'Brien has moved away from the sources and is describing what people may be thinking, or seeing during the course of Emma's life. Some of this is partially based on records, such as descriptions of the decorations on longships, but a lot is decent surmising. This isn't that much of a problem as you can easily spot where she's going off piste.
To be fair, there is a lot of good stuff in this book. It's particularly good on Harald Harefoot and Harthacnut, two kings who are generally skipped over in the same way that the sequels to Highlander are usually ignored – they're there, but not thought that interesting to talk about much. That doesn't mean that there's a huge amount to say about them here either, but there is more than in most books and the uncertainty is well brought out. When it comes to the authorship of the letters sent to Edward and Alfred, she remains on the fence over whether it was Emma or Harald Harefoot. This is not an unenviable position as it's like trying to decide between Dick Dastardly and Baron Greenback as to who's the most trustworthy.
O'Brien's a good writer and this book is an easy enough read. There are a couple of factual hiccups, but only in things that she mentions in passing. Ie, she gets the centuries wrong for the reigns of Cynegils and Ecgbert, is inaccurate in the comment about there being no prisons in Anglo-Saxon England (Laws of Alfred, Aethelstan, Juliana's poem, etc) and she does take an ASC entry of the enemy in the East our soldiers in the West, enemy in the South our soldiers in the North, rather literally. I was a bit surprised that there no mention of the putative raid on Normandy by Aethelraed, but ce la vie.
Three things you'll take away from this book:
1, Emma probably wouldn't have won mother of the year at any time
2, how small the political world of England, Denmark and North West Europe was
3, Edward the Confessor probably didn't smile a lot