Appropriately considering the source, more than a dozen color-splashed spreads precede the title and credit pages in the new National Geographic book "On Assignment USA." As with the society's magazine, photographs of great landscapes and incredible faces capture the eye . . . but the images are matched by an enlightening text.

"In its 110 years the National Geographic has published hundreds of articles on life, nature and science in the United States," Priit Vesilind, a staff writer for the magazine and this project, notes early on. "It has dispatched artists, writers and photographers to record the nation's progress and changes through time. In so doing, National Geographic has become our nation's journal of record. Or to put it another way, our nation's family album.""On Assignment USA," a book-length expansion of what has been a regular feature in the magazine - giving readers glimpses into the effort that goes into gathering the photos and the information for particular articles - turns out to be more than just another beautiful coffee-table book. It is a retrospective look at both 20th-century America and at 20th-century Amer-i-can journalism.

That is, journalism with the National Geographic Society's unusually vast resources and distinctive perspective: The writer's point of view is almost always present.

"One guideline for a Geographic story is that the writer should use the first-person narrative: I went there; I felt this," Vesilind writes. " `But you have to earn the right to put yourself in the story,' said retired senior writer Brian Hodgson. `You have to do something, be involved somewhere. One way or another, you have to get your feet and hands dirty, get wet.' "

These experiences can range from the participatory (Vesilind himself tells of working on an Ohio River towboat and a photo shows him as a vendor hawking peanuts at a baseball game) to the dangerous.

In eight chapters on such subjects as "Land," "People," "Cities" and "Progress," Vesilund focuses on the production of specific stories for the magazine, by himself and by others. For instance, when Noel Grove was writing the 1975 article "Mark Twain: Mirror of America," he decided to build a driftwood raft and float down the Mississippi.

"It was scary," he told Vesilind. "That river is so powerful. On the last day, a young photographer from the Des Moines Register and Tribune jumped on the raft to take pictures. We tried to get back to shore, and we could not. The current would not let us. When I saw his photographs later, the fear was in my face. It was sort of twisted - not a pretty sight. Was I going to drown three people here because of my own foolishness?"

Although journalism is journalism, in some ways National Geographic articles differ from those in the daily press and in weekly magazines. While coverage can sometimes seem belated in the Geographic, with stories appearing months after a hero's death or a hurricane's devastation, generally time is on its side. The magazine's writers and photographers spend months on a single story; other journalists are lucky to spend hours, maybe a day or two or perhaps a week at most on the same topic.

But the interaction of the Geographic's writers and photographers with people in all walks of life - however fleeting those encounters may be - are in many ways typical. And their observations are on the mark.

National Geographic specializes in a variety of journalistic categories - stories about journeys, places, cultures, scientific discoveries and accomplishments, and about individuals and peoples, or "special communities."

The latter, Vesilind notes - "be they baseball players, cowboys, harvest workers, Native Americans or Amish farmers - can be both difficult and immensely satisfying for journalists. Writers and photographers are intrusive by definition, and they must work hard to build trust."

Journalists are chroniclers and witnesses, sometimes pesky, sometimes aloof.

In 1973 young Peter Gorton Jenkins talked the magazine into letting him write and photograph his way across America, walking. He told his illustrations editor, Thomas R. Smith, about seeing firsthand the fervor among the people in a rural black church.

"And Tom says, `Peter, are you taking pictures?'

"I said, `No, I was too overwhelmed to take pictures.'

"He said, `As a writer you can sit down later and write about these things, but as a photographer you have to become sort of detached from yourself. You have to stay above it. And don't let an experience pass because you think it will happen again. It almost never does.' "

On the other hand, the Geographic envoys say, they also should show discretion.

"I've walked away from pictures that I later thought maybe I should have taken," said photographer Bill Allard, "but I'd rather err on the side of the subject."

With its memorable scenics (including such Utah locations as Monument Valley and Arches National Park, and an aerial shot on the dustcover), creative photo-illustrations and insights into American lives, "On Assignment USA" nicely blends journalism and anthropology, personal stories and cultural impacts. The book offers glimpses into the lives and experiences - including the occasional hardships and inconveniences - of National Geographic's journalists and photojournalists.

" `So how was it?' friends and neighbors ask," Vesilind notes. " `What's it like to be out there?' I usually relate some funny stories, but I can't fully explain the experience of living another life. I tell them that in the field I'm the eternal stranger, and at home I'm considered temporary help, and sometimes I feel like an intruder in both places.

"But they don't want that answer: no griping allowed when you have the greatest job on earth."