Gwenhwyfar: The White Spirit

Gwenhwyfar: The White Spirit is a Historical Fantasy novel by Mercedes Lackey based on Arthurian legend.

The novel is inspired by a Welsh tradition that King Arthur had not one but three different queens, all named Gwenhwyfar (Guinevere). The protagonist of the novel is the third and youngest. Lackey also follows the Welsh tradition about Gwenhwyfar having a sister named Gwenhwyfach.

In the novel, Gwenhwyfar (nicknamed Gwen) was born as the third of four princesses to a Celtic king. Her mother has noticed that she has powerful potential for magic, but Gwen would rather spend her time with horses. Her father, who has no sons, encourages her to pursue her dreams, and Gwen becomes a warrior. Meanwhile, the High King Arthur is trying to unite his kingdom while striking a delicate balance of power between the Ladies and Druids of the Old Ways, and the newly arrived Christian priests.

The story follows Gwen from childhood all the way to adulthood, telling the stories of both her life, and the part she would play in Arthur's.

Tropes used in Gwenhwyfar: The White Spirit include:

Action Girl: Gwenhwyfar, obviously. Her childhood idol, Braith, also qualifies. The original Gwenhwyfar, Arthur's first wife, is likewise mentioned as having been skilled with a bow.
Anything That Moves: Anna Morgause, apparently.
Big Bad: Medraut
Blood Magic: Morganna, Anna Morgause, and Gwenhwyfach seem to use this sort of magic
Brother-Sister Incest: Arthur and Anna Morgause
The Chessmaster: Morganna has shades of this, though she's barely present in the story. Her nephew, Medraut, however, has numerous schemes and backup plans in the works.
Creepy Child: Medraut is mentioned as having been rather unnerving as a boy. Little Gwen, also had shades of this, especially as she became more aware of her ability to control men.
Damsel in Distress: During the final third of the book, Gwenhwyfar is kidnapped by the Big Bad and replaced with an impostor. Subverted in that Gwen takes advantage of a distraction to escape Medraut's tower, and ends up saving Lancelin-- who had come to rescue her.
Deal with the Devil: Subverted. Gwen's deal with the Annwn is actually favorable for all party's involved.
The Fair Folk: The Annwn can be absolutely merciless and are renowned tricksters. Fortunately for Gwen, they owe her a debt, and Gwyn ap Nudd treats her with the utmost respect
Four Is Death: Gwenhwyfach, or "Little Gwen", was the fourth daughter and Gwenhwyfar's little sister. She is also the pettiest, cruelest, and most manipulative of the four sisters.
Genre Savvy: The abbot, Gildas, realizes that Gwen has been replaced with an impostor almost immediately
He's Just Hiding: Arthur's second wife faked her death and went to live in the monastery
I Have You Now, My Pretty: When Medraut was a child, he announced his intention to marry Gwen... then he kidnaps her from Arthur's court many years later
Enfante Terrible: Even as a baby, there's something off about Medraut...
Merlin: Naturally. Like The Mists of Avalon, it's a title for the head druid rather than his name.
Nice Job Breaking It, Herod: Arthur and the Merlin conspire to kill all the newborn infants in the kingdom at one point in an effort to kill Arthur's illegitimate son (and nephew), Medraut
No Guy Wants an Amazon: Gwen is warned that few men will be able to see her as both a woman and a warrior. And indeed, this seems to be true (especially where Arthur's concerned) Lancelin turns out to be able to see her as both
Spell My Name with an "S": Most of the characters have names that are different than how most people would recognize them: Gwenhwyfar instead of Guinevere, Medraut instead of Mordred, Lancelin instead of Lancelot... In most cases, this is because the author is using the original Welsh spelling. Not for Lancelot though, as he was most probably invented by French authors.
What an Idiot!: Arthur dismisses Gwen's speech about how she is a warrior as just being hormones and assumes that she is pregnant. Then he proceeds to not even notice when Gwen is kidnapped and replaced with her pettier younger sister
What Happened to the Mouse?: Morganna just seems to drop off the face of the earth halfway through the book