Cooper has made copious use of the Matter of Britain, the cycle of stories centering on King Arthur. She lists as one of her primary sources the Welsh Mabinogian (circa tenth to fifteenth centuries), which probably predates all other stories of Merlin and Arthur. Both C. S. Lewis and J. R. R. Tolkien, who were Cooper's teachers at Oxford, used these materials in their works of fantasy.

But Cooper's blend of ancient, medieval, and modern elements bears her own unique stamp and ranks as an imaginative achievement of the first order.

Cooper's use of the Welsh language adds authenticity to the narrative. Part Welsh herself, she exhibits a keen instinct for the sound of the language and has researched the accuracy of her character's phrases. In addition, when Bran gives Will a lesson in the pronunciation of Welsh words, readers gain this knowledge as well. Although...