Radiance Is in the Details : A SOLDIER OF THE GREAT WAR, <i> By Mark Helprin (Harcourt Brace Jovanovich: $24.95; 695 pp.)</i>

Into his freighter of a book, Mark Helprin has loaded some of the best magic around. Also his riding boots, his mountain boots, his shining grasp of childhood, a haze of inspirational rhetoric, and metric tons of material about various kinds of infantry fighting in the First World War.

It is a vessel of surprised delight, but what a cargo the delight is packed in with: howitzers and other conventional war supplies--well made, well secured, ponderous--and a deal of unsecured ballast that shifts to cause lurching and the loss of headway.

The reader needs dedication and wharf room to unload and sort it all; to separate what is magically transformed from what just trots along from what just lies around; to winnow Helprin’s play from his gamesmanship, his intoxicating side glances from his ponderous head-on stare.

“Soldier” is told by Alessandro, an old Italian, to Nicolo, a young one. Having, respectively, been thrown off and missed a bus from Rome to Monte Prato--for satisfyingly improbable reasons--they decide to walk the 45 miles. Alessandro decides, that is; Nicolo wonderingly consents. The old man will use the time and the exploit--they will be his last--to transmit his life, and his vision of what life is about, to the younger one. He needs, as in the myths of death and resurrection, an adept. The adept needs a mentor.

Alessandro, son of a loving and prosperous Roman family, tells--we don’t see him telling; he grumbles to Nicolo that he needs all breath for walking; his story gets imparted, nonetheless--of a life that begins idyllically before the turn of the century, is shattered by the First World War, and rebuilds itself precariously afterwards, its Edenic joy replaced by a joy shaped out of pain.

“The spark of life,” he tells his companion, “is not gain nor is it luxury. The spark of life is movement, color and love.” This is Alessandro’s theme through the horrors and splendors he undergoes; it is also a sample of Helprin’s skids into sententious lyricism.

Alessandro’s story begins with a childhood trip with his father to the Alps--then held by Austria--where he falls in love with a little girl who is a member of the Imperial family. It tells of his youthful pursuit, partly on horseback, of a beautiful young Roman neighbor. It goes on to tell of his grim wartime service, first in the trenches, then against Mafia bands in Sicily, finally in the mountains against the Austrians.

It tells of his desertion, apprehension and impending execution, and of his last-minute reprieve and his brutal prison labor quarrying marble for gravestones. It tells of his capture by Bulgarian irregulars, and later by an elite Austrian detachment. It tells of his shattered family, of his love for an army nurse, of his despair when she apparently is killed in a air raid, and of his seemingly deranged and ultimately rewarded effort to seek her out after the war.

It is a life that is hellish and radiant by turns. And it is Helprin’s unique quality that, when he does not fall into rhetoric and didactic uplift, he is far more original and captivating in his details of radiance than in those of hellishness.

It is hard to define Helprin’s elusive power, which was also evident--along with the unevenness--in “Winter’s Tale,” a novel of New York as grand historical myth. It claps together comedy and sudden beauty, as if the absurd, in all its unexpected details, were a gateway not to skepticism but to wonder.

The first episode is a superb example. The child Alessandro finds himself in a place so astonishing that it seems magical. It is an alpine lodge sited so precipitously that all its supplies reach it by a cable car that ascends vertically for a half-mile. The arrival of the Imperial party turns the lodge’s routine into an affair of stunning sumptuousness, with a splendid military escort, rich food, a band, and the golden child who enchants the boy.

That night, a bandsman has a heart attack and must be taken down by cable car, but his chest must be massaged on the way. Alessandro is the only one small enough to fit. He is strapped in and plunges through a starlit void. It is so wondrous that it seems entirely natural, when he returns, for him to creep down the hall and find the room belonging to the royal child. Impressed, she welcomes him companionably into her bed.

The fairy tale quickly turns into farce when they are discovered. Alessandro’s father slaps him--for the first and last time--so that the Austrian officials will not do worse. The two make a hasty departure, and the father apologizes tenderly on the way down. But farce and tenderness do not end it. The last image is the wheel on the mountain top revolving continually to run the cable. Does it turn even when nobody is there? the boy asks his father; and when told that it does, he says: “Then I am not afraid to die.”

Alessandro’s courtship of his equestrian Roman neighbor, Lia, is wonderfully funny and exhilarating, culminating in a splendid horseback chase over the Roman countryside. Another superb episode comes when Allessandro is taken prisoner by the Imperial Hussars. It is a most unusual outfit, he discovers. Commanded by Field Marshal Strassnitzky--his rank is absurdly elevated to make him worthy of the Emperor’s special troop--it spends its time galloping madly in all directions and eating gourmet meals in the field.

What it does not do, Alessandro discovers, is fight. Strassnitzky invents battles in his reports; he even invents imaginary soldiers of his own so he can report casualties--bloodlessly. He is a pacifist, he explains. “The object of war is peace and I have merely taken out the middle.”

There are other episodes, equally winged in comic irony or comic ravishment and sometimes both.

In contrast are the episodes of war in trenches and mountains, and Alessandro’s desertion and long punishment. They are the book’s center of gravity; they provide the dark ordeals from which the protagonist’s vision of life as heroic chiaroscuro will emerge. But here, Helprin’s special qualities show themselves only rarely. The bloodshed, the horror, the mountain climbing, the exploits are painstakingly written but they seem effortful, and, in all their detail, interminable. They are set scenes that have been set, perhaps, too often in literature.

Finally, they are an unassimilated weight in the heart of the book. With all Helprin’s magically realist gifts, they are plain realism, and when the author tries to elevate them into epic dimensions, they turn dutiful.

Helprin’s genius is for sending powerful charges through what seems peripheral, unexpected, elusive. Even in Alessandro’s loves, it is the odd or fleeting encounters that are memorable. His great central passion for the nurse Ariane is virtually invisible. His quixotic postwar search for her has some magic, but when he finds her we don’t see her. He marries her and we still don’t see her. Nor do we see him seeing her. Nor do we see Helprin seeing either one.