‘Memory is both a duty and the subject of his artistic work,’ writes Cambodian artist Karay about Vann Nath (above). Photograph: Heng Chivoan/Phnom Penh Post

Apile of off-white skulls with dark, hollow eye sockets sit at the base of a large tree, its roots growing like tentacles. Nearby, a woman kneels at a shrine, remembering the dead.

The subject of this painting, like much of Vann Nath’s work, is memory.

It’s also the common thread that weaves through 15 reflective, previously unpublished essays that appear in Vann Nath Tribute, a new book dedicated to the revered artist – one of only a handful of Tuol Sleng survivors.

While imprisoned at the notorious Khmer Rouge torture and execution centre known as S-21, Nath was forced to paint pictures of Pol Pot and other senior Khmer Rouge leaders.

Later, as a survivor, his paintings documented in graphic detail the horrors inflicted on his fellow prisoners.

“Memory is often manipulated because it is fragile. The unshakeable reconstitution of Vann Nath gives strength to his own memories,” writes the anonymous Cambodian artist known as Karay in his essay.

Nath, the first witness to take the stand at the Khmer Rouge tribunal, testified against his former jailer, Tuol Sleng commandant Kaing Guek Eav, better known as Duch.

It was not the first time that Nath would face his former tormentors. He made a searing appearance in Rithy Panh’s film S-21, confronts his former captors in his search for answers as to what happened and why.

Karay writes that Nath’s role in that film conveyed “his obsession with the search for truth, the constitution of the facts in all their horror”.

The new book, published as part of a tribute exhibition held earlier this year, features paintings from Nath and artists he inspired alongside prose written by friends, critics, writers, artists and historians, including notable tribunal figures David Chandler and Helen Jarvis.

“He was an inspiring and thoughtful witness to a pitch-dark period of Cambodian history,” writes historian Chandler.

“He was never willing to dig a hole and bury the past. Instead the past lived inside him, every day, and he bore witness to it, courageously but with an accessible, compassionate humanity as well.”

This commitment led Nath to wear many hats in the last years of his life. He was also a teacher, a patient interviewee for scores of journalists and a writer who produced the only firsthand account of S-21.

Panh notes in his introduction that while many expected Nath to be a civil party at the tribunal, he instead chose to be a witness, a move that was indicative of his character.

“He made it clear that he did not only want to tell about abuses; he was ready for a confrontation of ideas, for moral confrontation . . . ”

The book is published by the Friends of Vann Nath, a group originally established to raise funds to pay Nath’s medical expenses during his fight with chronic kidney disease.

Since his death in 2011, the group’s mission has been to keep Nath’s memory alive.

Yvon Chalm, president of the group, said Nath was especially concerned that Cambodia’s younger generations would not understand the magnitude of the genocide experienced by their parents and grandparents.

“He knew he would die and that the future of Cambodia was more important than his own life.”