Could the Grail story have meaning in a context other than Medieval Christianity? This is a serious attempt at finding such meaning.
In the rest of this review, there will be spoilers.
Kingfisher shares some themes with McKillip's other recent novel Solstice Wood. But whereas that was set in our own world, this has a peculiar setting furnished with automobiles and asphalt but ruled by royalty and feudal nobility. The setting is the novel's greatest weakness: there is simply no way it can make sense. All superficial aspects of modern life are there, from tourism to credit cards, but there is no apparent industrial basis for high technology (no mention of industry at all), no noticeable economic foundation for the kingdom, and no explanation how its modern economy can coexist with a feudal network of small principalities whose rulers chafe to be independent of the king. The king is served by knights but it is unclear what governmental or social function they have; they don't seem like a very useful army, with their skills (however formidable) all shaped toward single combat, and there is no mention of any other army in spite of rumors of military threats. We expect the workings of a fairy-tale kingdom to be vague, but in this novel, things are so concretely described that they demand explanation. Therefore, the failure of the attempt to combine feudal structures and modern trappings means that I'm not convinced that royal courts can fit into a modern mythology such as the one this book tries to construct.
Much more successful is the web of meanings McKillip weaves around the Grail itself. It is many things to many people, in a world of many gods. The kingdom of Wyvernhold has its capital city in Severluna, at the confluence of rivers named for Severen, the god of metals, treasure, and weapons, and Calluna, the goddess associated with the moon and births. We find ourselves in the territory of conflict between matriarchal and patriarchal religion that is familiar from many modern Arthurian stories. The theme is developed with so much nuance and detail here, though, that it doesn't feel entirely hackneyed or obvious. The acolytes of the two gods each have their own, sharply different version of a myth involving the Grail: the devotees of Severen say it's a golden treasure that's his by right, with Calluna trying to steal it, whereas Calluna's myth (which the author will give the right to) has her healing Severen.
Furthermore, it turns out that this is a divided realm, the fairyland of Ravenhold, ruled by queens, having been suppressed by the conquering kings of Wyvernhold, but the two still parallel each other in a shadowy way. Both lands are damaged, with Ravenhold having fallen into ruin and Wyvernhold into a state of disenchantment signalled by the extinction of fabulous beasts such as wyverns; several people speak of feeling a sort of spiritual exhaustion (and the Guinevere/Lancelot story seems to be playing its traditional role of being a sign of spiritual trouble). The royal families of both realms are seeking to recover the Grail which they both believe would give new power to their reign. There is a story naming as the origin of evil Ravenhold having allowed itself, for the first and last time, to be ruled by a king; this king, after having brought ruin to Ravenhold, at which time the Grail was lost, continues to spread corruption in Wyvernhold. This origin story is tied in a complex way to the link between Calluna and Severen.
The Grail itself, the overflowing source of life and nourishment, plays a role outside these conflicts; it seems more at home in the "castle of the Fisher King" (currently serving as a fish fry restaurant) than in the hands of any power. Yet more meanings are attributed to it by the knights who seek it with a variety of personal attitudes and motivations, and of course, the possibility of the success of their quest is in their heart. The major elements of the Perceval story are repeated in this novel, but some of them are repurposed. Pierce Oliver, hidden from the world by his mother, but tempted toward the court and knighthood, is Perceval (with elements of Galahad in that his father is Lancelot) and a naive "fool" character; he is the "kitchen knight" whose skills are cooking not fighting. He has the usual complement of women that accompany a knight in such stories, but playing unusual variations on their roles: his sorceress mother (who saves him from a dangerous encounter on the road rather than him saving himself); a sister-figure, who assists him in his fight (but who has her own story that ultimately has little to do with him); and a damsel in distress (who takes his kitchen knife away from him and does in her captor with her own hands). His ultimate role is to bring people together, so that they can deal with the evil in the land, rather than to do anything active. And he accepts that at the end, saying that he's a cook rather than a knight, and the restorative, fellowship-promoting power of food is what he appreciates.
The ritual procession that Pierce/Perceval witnesses at the Fisher King's castle is as in the original legend. What we learn of the past suggests that these people are the true heirs of Calluna and old Ravenhold. The one element of the story that doesn't fit very neatly into this telling is Perceval having failed to ask a crucial question; that is brought up early on but not carried through to the end. However, the novel ends with the Fisher King being not yet healed. That's because the true damage is the estrangement between Ravenhold and Wyvernhold. There are signs of reconciliation, and people on both sides talking about the necessity of it, but it hasn't happened yet, and will presumably be an extended process. That's where the similarity to Solstice Wood lies: that novel considered the traditional view of fairyland as separate and hostile and suggested that in modern times that view might break down, that due to cultural shifts people might have reasons for not fearing and rejecting it any more.
Kingfisher is a complicated, many-charactered, somewhat problematic but ultimately quite appealing work whose themes of the reconciliation of different myths and different worlds of the imagination is indeed timely.