I'm not sure if this was Holdstock's last book, but it was certainly his last book about Ryhope Wood, the first being the justly famous Mythago Wood. It's interesting to compare first to last. Mythago Wood starts off as science fiction, or at the very least with a character who is trying to understand mysterious phenomena in the ancient woodland next to his home scientifically. Before Avilion is over it seems like nothing but magic can explain all the bizarre goings on. Mythago Wood is very much about mythic archetypes. Avilion is very much about specific characters from myth/legend. There's a big difference between a warrior who unites a kingdom and Arthur who dies fighting Mordred; the latter is a specific instance of the former. Mythago Wood is about a family that breaks apart self-destructively. Avilion is about a family that despite separation, remains a strong, healthy unit. Mythago Wood is about outsiders entering an alien realm. Avilion is about people who have an inside perspective of the same realm.
When I think back over the several Holdstock works I've read, I notice a common theme of writing about broken families - families that have become physically or emotionally separated (or both), families where internal abuse of power occurs. Families that either struggle to repair themselves or dissolve into anger and hate or get their members hopelessly lost to each other in space or time or emotional distance. Interesting then, that this final Mythago book lays heavy emphasis on hope for the family that have been central to the entire saga. Unfortunate that it does so too heavy-handedly, at the end. Also unfortunate that the plot sags before the denouement, taking too much time to move the chess-pieces (aka characters) to their correct spots.