Anne Fine is on her best form here, in what must be one of the best stories about teenage friendship ever written.

Ian and Stol (Stuart Terence Oliver) are more like brothers than friends. Ian is the happily adopted son of loving parents, who in practice have almost adopted Stolly too. Stol’s birth parents, a barrister and a fashion designer, are too wrapped up in their careers to be more than negligently loving, and are happy to park their son with Ian’s parents.

Stolly is eccentric, a gifted one-off. The story takes place in the hospital ward where Stolly is lying semi-conscious, recovering from the latest of many accidents, this one caused by falling from a high window at his home. Ian, helping to keep vigil at his bedside, writes down the story of Stolly’s life so far, attempting to make sense of his friend’s mercurial, self-destructive nature.

Stolly knows heights of happiness and depths of wretchedness such as ordinary boys like Ian do not. His mind is independent and original, good on lateral thinking, fearlessly outspoken and alarmingly truthful. But all this zany brilliance has its price in impulses of flippant, suicidal despair, what Ian calls “a kind of cosmic exasperation”.

Ian is Stol’s saviour, and the book’s triumph. Well-adjusted, caring Ian could have been just a saint or a cipher. Instead, he makes normality interesting, and his lively mind prevents the novel from turning into a psychological casebook.

Steering by Ian’s bright, sane star, Up on Cloud Nine is an unforgettable and cheering tragi-comedy of teenage bonds and loyalties.