'Percival's Angel' is set in the same world as 'Merlin's Harp,' (see my review of that book for feelings that apply equally to this novel) but it works very well as a stand-alone novel. Readers will benefit to a degree from having some of the background of the world, and backstory on minor characters if they've read the previous book, but it's not at all necessary.
The story is loosely based on the tales of the Knight of the Round Table, Percival, and the quest for the Holy Grail - but it's really about clashes between cultures, conflicts between people, and the search for meaning in life... like so many 'quest' stories.
The novel follows Lili, a girl of the fey, as she follows her childhood friend, Percy, out of the protected forest where her people live in isolation. Percival's mother took him into the forest as a babe, hoping to keep him from ever becoming a knight - a calling that proved the death of all her previous children. Regardless, Percival seeks to break free of his mother. Lili seeks to help him, and discover the power of the human heart, which Merlin has told her about, and which, as a member of the fey, she feels that she lacks.
Soon enough, Percival is recruited into the Quest for the Grail... which is simply a story that Merlin made up, to keep troublesome knights occupied in peacetime. But where humans believe, power is lent...
I didn't like this book quite as much as Merlin's Harp, but it's still a beautiful work of mythic fiction, rich in layers of meaning.
I liked it enough that, as soon as I finished it, I ordered Crompton's third book based on Arthurian tales (Gawain).