The White Mare is a work of historical fiction, with dabs of fantasy and a strong dash of romance. The setting: first-century Scotland, as the Romans move to conquer lands traditionally held by the peoples we know as the Picts. Irish exile Eremon arrives just in time to be appointed war leader by a local tribe; their princess and priestess, Rhiann, is pressured into marriage with him despite her wishes (and from there, the story goes exactly where you think it does). It should be noted that this is an odd sort of trilogy: the first two books are meant to be read together (this one ends on a bit of a cliffhanger) and then there's a 300-years-later sequel.
I'll admit that when I first started this book, I was not impressed. The writing style is merely passable, with a tendency toward melodrama. The characters are nothing original, and some tired tropes are dragged once again into the light: the family wiped out by raiders in the backstory; the character who always behaves "honorably," even against their own interests, without any compelling reason to do so. And yet, as I read on, the book began to have a certain charm for me.
From the beginning, the author does an excellent job in her depiction of everyday life in ancient Scotland. The backdrop here is vivid, with lots of visual imagery, and the author brought her tale to life by showing how people spent their time. The plot is fast-paced, with lots of short scenes--too short at times, but rest assured that the story does not bog down in description. I became so eager to know what happened next that I read the entire second half of the book in one day. And--this is probably why I finally warmed up to it--the romance was unexpectedly romantic and touching. My only issue was with its resolution--forced rather than arising naturally from the characters; it just felt wrong on so many levels.
On the historical accuracy: very little is known about the Picts, so Watson invents as she pleases. The Roman invasion really happened, but many elements--like the Goddess religion and its priestesses--have no basis in fact, which may bother historical purists. As is common in prehistoric fiction, the religion is "real" within the context of the story, with some supernatural occurrences blurring the line between historical fiction and historical fantasy.
Comparison with Juliet Marillier's Bridei Chronicles is inevitable, since both are historical fantasy-romance focusing on the Picts. Watson's books are faster-paced, with more of an emphasis on war and more vivid descriptions of daily life. Marillier's books are slower, more character-driven, and being set a few hundred years later, have perhaps a more medieval, rather than prehistoric, feel. Readers will have to decide for themselves which trilogy they prefer, but if you're looking for a light read with war and romance in a historical setting, you could do worse than The White Mare.