The unseen victims of no man's land

Pat Barker's Regeneration trilogy did both. Not only did she make us stand in the mud, the filth and the blood, she told us what war does to the body and the mind of soldiers, in particular a soldier called Billy Prior. The trilogy, published in the 1990s, opens with Prior at Craiglockhart, the hospital in Edinburgh where soldiers with shellshock were treated; it was where the doctors made the patients well enough to die.

In returning to the first world war for her new novel, Toby's Room, Barker has produced much the same horrifying, hypnotising effect, although it is by no means a repeat of Regeneration. She certainly shows us what a shell or a bullet can do to a body but in killing off her central character before the book even begins, Barker also deals with one of the other great questions of war: how the survivors and the families cope; how they deal with not knowing what happened at the frontline, and sometimes not even having a body to bury. In the case of Toby's sister Elinor, the only remnant of her brother is his room at home. She lays out his uniform on the bed: "and there he was, his body shaped by the clothes he'd worn in life".

Because these after effects of the war are the main focus of Toby's Room, there are no great stretches spent at the Western Front, although there are some passages that take us there and – like all great first world war novels should – they provide images that lodge in your mind like shrapnel. The train journey of Elinor's friend, Kit Neville, to the frontline, for instance: "The thought of what was facing them kept him and others like him awake. Looking around, he could see, here and there among all the blank and shuttered faces, a glint where sleepless eyes caught the light."

It is sequences like those, in which men, all small and thoughtful, are caught in the middle of war, all huge and thoughtless, that give us the great moments of first world war literature. The novel that still does this best is All Quiet On The Western Front, Erich Maria Remarque's story of a group of 19-year-old German schoolfriends who volunteer to fight in the first days of the war.

The book, published in 1929 and based on Remarque's own experiences at the German frontline, is full of pictures that linger, like the shell that blows a cemetery apart ("they have been killed for a second time") and the butterflies playing around the trench ("I wonder what could have brought them here? There are no plants or flowers for miles"). Like Michael Morpurgo's War Horse, the novel also directly addresses one of the other horrors of that conflict: what it did to animals. "I tell you this," says one of the soldiers in one of its most harrowing sequences in which horses slowly die, "it is the most despicable thing of all to drag animals into a war."

One of the greatest of the British novels of the first world war, Siegfried Sassoon's Memoirs Of An Infantry Officer, which was published the year after All Quiet On The Western Front, is also filled with these ugly images, beautifully written: a pair of hands, for instance, that Sassoon says protrude from the soil like the roots of a tree turned upside down, or the dead lying by the side of the road, "their fingers mingled in blood-stained bunches, as though acknowledging the companionship of death".

However, what Sassoon's novel, a fictionalised version of his own disillusionment with the war, also does is perform the important trick of writing about men's emotions without ever explicitly addressing them. It does this because that is what it was like – the men didn't talk about their feelings and their fears; they talked about everything else instead. Which means you won't find any great words of angst or introspection in Memoirs Of An Infantry Officer, or RC Sherriff's great first world war play Journey's End, or Pat Barker's novels; but what you will find, in the gaps between the words, is what the war did to those who fought in it.

It is this understanding and subtlety that excludes some acclaimed books, such as Hemingway's A Farewell To Arms and Birdsong by Sebastian Faulks, from the list of great first world war novels. Both of those books are essentially archetypal love stories with the war as a kind of mud-encrusted backdrop; they never feel like they get into the deeper levels of the conflict and its effects, or ask the questions that something so random and confusing as trench warfare raises.

Toby's Room does raise those questions, the most important of which is: what makes a person? Is it the body? The face? Is it what they own or what they do? To some extent, all the characters in the book have to face those questions – in Elinor's case when she hears the details of Toby's death and how he was blown apart by a shell. "What she couldn't grasp was the idea of a human disintegrating; nothing left, not even a pile of greasy bones. And in only a second. Painless, everybody said. Yes, but also inhuman. Outside the natural order of things."

Elinor's friend Kit also faces the same questions but in a much more direct way when his face is horribly damaged by shrapnel. His features become a kind of no-man's land of flesh, hidden behind a mask. The question for him is: if you lose your face, do you stop being you? Do you become someone else?

There is a lot more of this body horror in Toby's Room – in fact, in many ways it's a novel obsessed with bodies, and not just those of dead soldiers. There is also the naked body, the injured body, the body as art and the body as an object of sexual desire (as in Regeneration, Barker doesn't avoid the eroticism of war, and the homo-eroticism of it. Indeed, in Toby's Room, it provides one of the big twists of the book.)

What Toby's Room never does, though, is find any meaning to the war, because it can't. No first world war novel ever can, and it's another reason for their power: all this death and pain and horror, and it doesn't mean anything. In All Quiet On The Western Front, the schoolboy soldiers call it a kind of fever; in Memoirs Of An Infantry Officer, George sits by a candle and decides that the immense destruction can never be explained ("what I feel is no more than the candle which makes tottering shadows in the tent"), and in Toby's Room there is only the churned-up flesh, the torn minds and one man who is no longer here.

Toby's Room is published by Hamish Hamilton at £16.99. Pat Barker is at the Edinburgh International Book Festival on Friday at 1pm. Go to www.edbookfest.co.uk or call 0845 373 5888