Okay, so for readers of children's fantasy this is huge. Enormous:

There is a new Alan Garner novel. It is the sequel to The Weirdstone of Brisingamen and The Moon of Gomrath. It has been literally decades.

The novel is entitled Boneland, and will be published in late August in the UK. Not sure about a US edition. Garner has, as I understand, moved away from publishing explicitly for children, and it sounds like this book ages the characters at least roughly comparable to the time between novel releases, so this may not be specifically a children's book. Here's a blurb:

A major novel from one of the country’s greatest writers, and the crowning achievement of an astonishing career, BONELAND is also the long-awaited conclusion to the story of Colin and Susan – a story that began over fifty years ago in THE WEIRDSTONE OF BRISINGAMEN…

IF THE SLEEPER WAKES, THE DREAM DIES…

Professor Colin Whisterfield spends his days at Jodrell Bank, using the radio telescope to look for his lost sister in the Pleiades.

At the same time, and in another time, the Watcher cuts the rock and dances, to keep the sky above the earth and the stars flying.

Colin can’t remember; and he remembers too much. Before the age of twelve years and nine months is a blank. After that he recalls everything: where he was, what he was doing, in every minute of every hour of every day.

But Colin will have to remember what happened when he was twelve, if he wants 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