Gwenhwyfar: The White Spirit. An Arthurian Novel.
And the cover art shows a sword and a shield.
A few more hints: Gwenhwyfar is more commonly called "Guinevere".
That's right: this story follows Gwen, a warrior-princess who later becomes the High King's third wife (somewhat unwillingly). In this novel, there are plenty of swordfights, plenty of magic, a shitload of horses, elements of typical Arthurian legend, and a strong (female) lead character.
A fun read indeed.
(The picture on the left was totally lifted from this, because the other images of the cover Google showed me weren't identical to the cover of my library's copy. And I suddenly decided to care.)