Rachel Kelly, renowned artist of Penzance in southwest England, is dead. After a constant struggle with bipolar disorder, she leaves behind a Quaker husband (her anchor to sanity Antony), two sons (the insecure Garfield and mildly hedonistic Hedley) and a daughter, Morwenna, who inherited her fractured genetic code.

There was another son, Petroc, the youngest of the children and the one who most inspired her, but he died years earlier.

For Antony, who 'cried and raged and brooded' over his wife's body after finding it in her attic studio, 'what kept betraying her absence was the unaccustomed tidiness everywhere. And the calm.'

In the flood of inquiries after Rachel's death, it becomes clear that there's a gap in her life story, specifically the years preceding her arrival in England from Toronto, which she never spoke about.

Antony finds a degree of solace searching the internet to unravel the mystery. With him is Hedley, whose presence is 'deferring for both of them a time of necessary recognition, perhaps even the time of full mourning'. He sorts through the physical remains of Rachel's life, discovering an extraordinary six-part abstract, later to be known as the Stones Sequence.

Notes from an Exhibition is structured as a catalogue of Rachel's life, each chapter beginning with an item - an oil on canvas; Indian ink and watercolour on paper; oil on board; red chalk on paper - that serves as a visual and contextual clue to a facet of her complex character.

It's a clever and convincing device, keeping the reader at a respectable distance from the disturbed Rachel and her chaotic surroundings, while gently drawing us into a state of empathy and admiration.

'Fisherman's Smock (date unknown). Cotton.' A note explains that the garment, bought at a local chandlery, was her preferred protective attire when painting and that she kept chocolate biscuits in the pockets. 'She was wearing an even more torn and splattered example the day she died, and was buried in it.'

Gale uses the memories and feelings of Antony and the children to add more detail and shading to a portrait of the artist's insanity. Gradually, the memories begin to coalesce into a coherent, if abstract, picture.

He tackles some difficult topics - bipolar disorder as a trigger to genius and the urge to drug it into submission; the meditative introspection of Quakerism as an antidote to selfishness; and the dedication and suffering required of the true artist to translate an image from the mind into a more substantial medium.

There's an underlying harmony here, and Gale achieves his objectives with elegance and wit. His mastery is the careful and measured placement of some breathtakingly beautiful passages. This is art, pure and simple.

Hauntingly, the absent Petroc asserts his presence through the stories of Rachel's other children, who had 'grown up thinking of her as mad first and their mother second'.

The notes tell us that when Petroc died, Rachel didn't suffer an expected mental breakdown but used a former barn door to produce 'a vision of black so intense it seems to absorb all the light in the room'. Gradations of darkness can be discerned. It's 'a painting of a Cornish night, complete with trees and cloud-muffled stars and, deep in the darkness, a lane'.

Her friend, Jack Threscothick, believes she 'needed to recreate in the viewer the sensation of her mind's desperate searching for meaning in the face of over-whelming loss'.

Gale has always been a fine writer - he has written 13 novels since his 1985 debut, Aerodynamics of Pork, which was quickly followed by Ease - and his readers have appreciated the evolution of his style and technique.

Rough Music (2000) and A Sweet Obscurity (2003) marked him out as one of the sharpest and most honest observers of the innate dysfunction in every family. His previous book, Friendly Fire (2005), can now be seen as severing a thread with his past.

This book marks a new path - a breakthrough novel that should allow him to reach a wider audience.

An exceptionally fine piece of writing, Notes From an Exhibition brims with Gale's talent for observing the human condition and his sensitivity to the significance of life's abstractions.