WHEN WALTER Lord was touring the Solomon Islands for his new book on the World War II Coastwatchers. "Lonely Vigil," he decided to visit Gold Ridge.

Gold Ridge, in the center of Guadalcanal, was ghostly, heroes of the Pacific war who radioed their hidden caves and huts on Japanese-held islands. These were the mysterious helpers who rescued hundreds of American fliers and sailors, including one John F. Kennedy. Lt. (j.g.).

"What I'd really like to do," Lord told the old Guadalcanal hand who had been assigned to help him, "is visit Gold Ridge. See just what it looks like."

What he had in mind was a 10-mile jeep ride and maybe a sandwich at the scene.

"Right-o," said the old hand, a British colonial like most of the Coast watchers themselves. "How about day after tomorrow? That'll give you a day to get ready."

"Get ready?"

"Sure. You'll have to buy the rice after all."

"Rice?"

"Why, for the bearers, man"

It turned out that Lord was to outfit an entire expedition food, pots and pans, flashlights and matches for himself and two native youths.

There was no road to Gold Ridge. There was barely a path.

Lord was 55 then (59 now), tall, not in particulary good shape, not athletic at all. He was a scholar, a former business newsletter editor and advertising copywriter, a survivor of a desk job in secret intelligence for OSS during the War, and lately an author ("A Night to Remember," day of Infamy" and other best-sellers).

Strictly speaking, he hadn't had any serious exercise since his Princeton days. But he was nothing if not game.

"The only serious mistake I made was the biscuits. The old hand said they eat a lot of biscuits. I thought of course he meant those British-style biscuits, a bit sweet, so I bought box of petit-beurres. Well, on Guadalcanal, biscuits mean hardtack. But we managed."

How to reach the jungle? Simple. He hailed a taxi in the capital, piled with his supplier and his young guides Kasper and Nevi, and said, "To the jungle."

Five miles later the taxi pulled up where the road ended at a solid wall of green. "The jungle," announced the driver.

"Meet us here in three days at noon," Lord replied casually. And off he went.

"It was my first jungle," he said. "It was a bad Tarran movie. Huge trees with huge leaves, no sunlight, very wet. Mosquitoes. And slippery. You never saw Tarzan slipping around. It rained constantly, the hardest rain I've ever seen, and I was as wet as if I neatly took off my shoes and socks and rolled them up and elegantly carried them across. Kasper and Nevi looked at me with wonder. Then I realized we'd be crossing probably dozens of rivers, so after that I just sloshed through."

His sneakers survived the trip, but he lost seven toenails from steep downhill trekking.

"There's no underbrush in the jungle, no small game like chipmunks. You hear the birds but don't see them. Sometimes i daydreamed I was Nelson Eddy, sometimes Captain Spaulding the Groucho Marx character.All the jungle movies I ever saw went through my head."

He and his guides spent a night in a jungle village. They were joined by a third youth, who spoke English, which livened the conversation considerably. Lord had wondered who would cook - another hangover from the movies. To his faint surprise, he himself wound up as cook, because he was the only one who could read the labels on the beefstew cans.

(During the war, he noted, Americans believed the Japanese were natural jungle fighters, stealthy and cunning. The papers often spoke of their "simian" ways. But this myth was just a way of refusing to admit that it was their superior equipment that made them unstoppable at first. In fact, Japan has a temperate climate and Tokyo's weather is rather like London's, he said. And the Japanese soldiers, being simply kids from Osaka or farmhands, made just as much noise in the jungle as any other army, clanking and chattering as they marched.)

Lord found the Coastwatcher camp at Gold Ridge. He got a feel for the jungle and for the touch-and-go life those lonely lookouts led in the frightening days of 1942, when they risked death to warn the beleaguered Americans, "Forty bombers headed yours!" or "Six-inch gun on hill behind Lunga. . . ."

He saw a wilderness almost unchanged since the war, with rusting bulldozers and torn-off plane wings still scattered among the dark, deep thickets.

The reality he saw comes through in his book, the least-told story of World War II. These were men whose very existence was a secret all through the war, who were fictionalized in "Tales of the South Pacific" but mostly ignored - even though Admiral Halsey had said, "The Coastwatchers saved Guadalcanal, and Guadalcanal saved the Pacific."

In any case, their story is told now, and very skillfully. Lord found the research about as difficult as any he's done, for the subject is not listed in the archives as, say. Midway would be. He had to check the log of a submarine known to have taken some airmen off an island, then try to run down the names he found there. He roamed the Solomons for survived guides, laboriously tracking some individual. You don't simply phone up.

Gradually the looseleaf notebook he uses instead of cards fattened. ("I got that from J. K. Lasser, the tax man. I worked for him awhile.") He says he writes any old way, but his feat of organizing so much scattered material is remarkable.

"I got the idea first in 1947," said the bachelor Baltimore native, who now lives in New York. "I read a story in Michener about the Remittance Man, who was a combination of several Coastwatchers, I guess, and I wanted to find out what would make a man take on an islandful of Japs."

He did no more with the idea until 1966, when he was researching "Midway," and even then he decided to do another book first.

What next?

"I'm working on Dunkirk. I find it intriguing. I mean, usually you have an army to save the civilians from some enemy. But here the civilans saved Army."