"Boneland" is the belated and final part to Alan Garner's "The Weirdstone of Brisingamen" trilogy. However, it is not so much a conclusion as an exclamation mark at the end of the tale. As others have commented, the narrative style and themes of the book are much closer to "Red Shift" (my favourite of all his novels) and his later work than Weirdstone and "The Moon of Gomrath" and anyone expected it to flow seamlessly from the latter is likely to be disappointed.
Like "Red Shift" one of the central themes of the novel is the cyclical nature of history and myth. We all like to think that our life stories are unique, but really we are just repeating what has gone before many, many times. It is easy to read too much into that though and there is no need to presume that it implies reincarnation (though "Red Shift" does appear to suggest that) so much as just an understanding that human experience is shared through the ages.
"Boneland" follows the structure of "Red Shift" by having intertwining stories separated by time if not space. The first concerns Colin, some 50 years on from being the young hero of the first two Weirdstone books, and the Watcher, a hominid living, like Colin, on Alderley Edge but some 1-2 million years ago. The key to reading the novel is to recognise the parallels between the Watcher and Colin. This provides the code for understanding the significance of the first two parts of the trilogy and what that story means to Colin today. The lives of the two characters are linking by a hand axe (again echoing "Red Shift"), used by the Watcher to carve images in the rocks of Alderley Edge and now in Colin's possession.
The Watcher is shaman-like character who is unable to distinguish between the inner world of imagination and dreams and the external reality of ice, blood and hunger. The mythic reality notion of "As above, so below" is a meme that is central to the narrative and the realisation that the mind can create an alternative reality, expressed in stories and dreams, that can in turn influence and shape the outer reality is key to understanding what happened to Colin as a child.
The Watcher cuts the shape of animals in rock, and through those shapes he can connect, in his imagination, to their spirits. He is, though, the last of his tribe if not his species and tries but fails to draw a female to him by cutting the shape of a woman into the rock. Instead, at the point of despair, he is found by a group of the new interlopers, Homo sapiens, who provide him shelter and sustenance: "I sang and danced, and cut a woman for me to fetch a child for me to teach to dance and sing and cut. But you have come, not she." They listen to him with sympathy, but understand that his reality is different to theirs: "It is a true Story, said the other. It is a true Dream."
In 2012 Colin, now in his early 60s and a professor working at Jodrell Bank, is on the verge of a nervous breakdown. His erratic and eccentric behaviour causes alarm to his colleagues, his doctor and members of the public, and he realises he needs urgent help. His breakdown has been caused by unresolved issues from his childhood, when he suffered two major traumas in a short space of time. The first involved the sudden disappearance of this twin sister Susan at the age of 12. The second occurred shortly afterwards when he was struck by lightning on Alderley Edge. Lucky to live, the violence of the shock caused him to lose all his memories prior to that point, but may have been responsible for his genius-like intellect and perfect recall of everything that has happened to him since then.
Locked out from his true memories of his missing sister, Colin creates a mythic reality to explain her disappearance, and this is the story told in "The Weirdstone of Brisingamen" and "The Moon of Gomrath". In this fantasy realm, Susan is quite literally deified and her disappearance is a result of her ascending as a Goddess. Also in the fantasy, however, is another female character, the witch Morrigan. She is the diametric opposite of Susan, sinister and malicious compared to Susan's innocence and purity. This dichotomy sets up conflict within Colin, which needs to be resolved if he is to find a way to manage his childhood trauma. His inner self finds the solution by creating a third woman, the psychoanalyst Meg, a synthesis of Susan and the Morrigan, who is able to challenge and support him to confront his demons and ask the central question, who is Susan? He receives the answer he needs to be able to move on: "'Who are you?' he said. `You'."
Colin realises that the Triple Goddess he has created, Maiden, Mother and Crone, is part of him and will be with him always, and by understanding that he no longer has to search for the Maiden nor fear the Crone, he can stop hurting. The Susan in Colin's story was cut from his imagination and lost memories in the same way that the Watcher cut a woman from the stone: just a story, just a dream. But that doesn't mean that she is any less real, or the story she inhabits any less relevant, than any other part of Colin's life, and it has been a privilege to hear their tale.