Boneland, the conclusion to a classic fantasy sequence Alan Garner began over 50 years ago, will be published in August.

Garner, described by Philip Pullman as "better than Tolkien", launched his career in 1960 with the fantasy novel The Weirdstone of Brisingamen. One of the classics of British children's literature – although Garner has always denied writing specifically for children, he admits they understand his work better than adults – it tells of the adventures of the siblings Colin and Susan on Alderley Edge in Cheshire. Susan is the unknowing owner of the Weirdstone, a magical jewel sought by the wizard Cadellin, the "svart-alfar" (dark elves) and a brood of witches.

Drawing on local myth and legend, Garner continued the story of Colin and Susan in The Moon of Gomrath, in which the siblings light a fire on Alderley Edge and summon the Wild Hunt. The author has now completed the trilogy with Boneland, out from Fourth Estate in August, which sees an adult Colin searching for his lost sister.

"When I set out at three minutes past four on Tuesday the fourth of September 1956 to discover whether or not I could write, I saw the first piece of work as a trilogy, but I didn't know why that was so, nor what shape it would take," said Garner today. "Trilogies are strange creatures. The lack of the third book, I discovered, gave the readers of the first two a sense of urgency. There are nuggets in the text that hint of unfinished business. The links to the book-not-written had become subliminal cliffhangers. Why did it take so long for Boneland to gestate? All I can say is that it took as long as it took."

In Boneland, Colin is a professor who spends his days at Jodrell Bank "using the radio telescope to look for his lost sister in the Pleiades. At night, he is on Alderley Edge, watching", said Garner's publisher. "At the same time, and in another time, the Watcher cuts the rock and blows bulls on the stone with his blood, and dances, to keep the sky above the earth and the stars flying."

The adult Colin can remember nothing from his life before the age of 13, and afterwards he remembers everything, from every minute of every hour of every day. "Colin can't remember; and he remembers too much," said Fourth Estate. "And then, finally, a new force enters his life, a therapist who might be able to unlock what happened to him when he was 12, what happened to his sister. But Colin will have to remember quickly, to find his sister. And the Watcher will have to find the Woman. Otherwise the skies will fall, and there will be only winter, wanderers and moon …"

The publisher also unveiled a short scene from the novel. "A woman was reading a book to a child on her knee. 'So the little boy went into the wood, and he met a witch. And the witch said, "You come home with me and I'll give you a good dinner." Now you wouldn't go home with a witch, would you?' Colin stood. 'Young man. Do not go into the witch's house. Do not. And whatever you do, do not go upstairs. You must not go upstairs. Do not go! You are not to go!'"

Nicholas Lake, Garner's editor at HarperCollins, called the new book "a masterpiece. It's his crowning achievement, and in the manner of crowning achievements, it makes you reappraise is previous work. So there's a direct line to Weirdstone, but there's also a direct line to Thursbitch," Garner's 2003 novel for adults, "which means that it unites the strands of his writing in one brilliant capstone. I think people will be talking about it for years to come."

Although Garner had previously said, back in 1989, that a third book was "lurking within" The Moon of Gomrath, at the time he was sure it would never happen. "At one stage, before beginning The Moon of Gomrath, I thought it was a trilogy. By the time I'd finished The Moon of Gomrath, however, I knew it was time for me to be moving on. There was no need for me to write that third book. But there is, lurking within The Moon of Gomrath, the idea that something else is going to happen. And there is. There's a third book, which I shall never write. I couldn't write it now if I wanted to, because it wouldn't match. One changes," he told Raymond Thompson.

The author's multitude of fans – Garner's work is "where human emotion and mythic resonance, sexuality and geology, modernity and memory and craftsmanship meet and cross-fertilise", says Pullman, and "any country except Britain would have long ago recognised his importance, and celebrated it with postage stamps and statues and street names" – will welcome his change of mind.