The news came as a shock. The mail room workers down at Jimmy Carter's Transition City on the grungy fifth floor of the HEW North Building had finally caught up with a backlog of 100,000 letters and had gone home to enjoy the New Year's holiday weekend. They turned on their televisions and there it was.

There were Jimmy Carter telling all the country to write him and tell him how to avoid feeling "isolated" as President "in the strange and unnatural world of staff and press and politicians."

"Then," says Viola Harkins, the one and only original Carter letter-opener from the days when pen pals were scarce, then says Viola, "the address flashed on the screen - People, P.O. Box 2600, Washington, D.C. 20012 - and I looked and said, What the hell is he doing? Does he realize what he's doing?"

The address on TV freaked out everybody in the mail room.

"Shocking," says one.

"A monster," says another.

"I saw this immediate deluge of mail," says a third.

"I didn't know about it 'til I got to work the next morning," says still another, "and then I saw all these long faces in the mail room."

"Now wait a minute," says a Carter spokesman, "you're not going to make it seem that we don't want mail, are you? We want the letters."

They want the letters.

"It's being taken seriously, it really is," the spokesman continues. "It's no joke. We're having some very serious people read every letter that comes in - no matter how many there are."

The mail room already is whirling. Most of the 350,000 pieces of Carter mail since the election have hit there, including a one-day high of almost 20,000. They are attacked by a dogged team of sorters and openers, then passed to a still larger group in a larger room, silent huddled masses with heads bent, reading. They're seated in front of whole walls full of pigeonholes, with such handwritten labels as "job seekers," "autographs," "children," and "help," from those who say they need it.

Now, along with all the other mail, comes the first wave of "People" mail, about 3,000 letters this week. Readers along one wall are detached to take it: shortly it's announced that about 300 of the first 1,500 letters read contain "constructive suggestions."

It's suggested that Carter have over some "just plain people" for lunch or dinner at the White House: that Carter and Mondale "meet periodically in public school auditoriums . . . in old-fashioned town hall meetings": that the Carters, Mondale, and Cabinet members "devote one Sunday afternoon to calling people all over the country for a three-minute conversation."

It's suggested that Carter set up a "toll-free hotline" and "people centers" and that he "leave the White House sometimes and share the life-style of average Americans," that he should experience the daily burdens of traffic jams and supermarket prices": that he not eat at "the real exclusive places like Sans Souci or Trader Vic's and places like that" but McDonald's, "where the people eat."

Steven Shoob is opening a mail sack right now in a room behind the big room with all the pigeonholes. He turns the sack upside down and all the letters fall out and then an object tumbles out and goes thunk on his desk. Shoob does not like objects that go thunk on his desk because several weeks ago he discovered a mail bomb. So he picks up the little package for Jimmy Carter and scrutinizes it. The contents are identified on the outside.

It's from Tennessee, four fluid-ounces of "Arnold's Liniment, recommended for the relief of arthritis and rheumatism." It gets thrown into the gift bin, a small room absolutely crammed with who knows what all, from a "Yokie" hand exerciser and a bottle of maple syrup labeled "A nip of northern comfort" to a hand-made red, white and blue silk chiffon gown for Rosalynn to wear for the inaugural. There are two one-dollar bills from a man who says he's been to outer space and back: stack of books, records and tapes, scores of them, some said to contain peoples' life histories.

Gifts, because they can be bulky and hard to move around, compound the problems at Box 2600. "Box 2600 is not really a box," says a Carter worker, "it's a lot of mailbags." They're loaded at the main postoffice and taken to the Executive Office Building where they are screened for explosives. Then Steven Shoob and his friend, Robert Shipley, drive over and get the sacks.

They used to screen the mail themselves. But one day Shoob saw a suspicious item and had everybody clear the area while the bomb squad came in. It turned out to be a doll. A couple of days later Shoob detected another package he didn't like and had everybody clear out. "Everybody went out singing and humming," says a worker. That time it was the bomb. "Right in that corner," says Shoob, pointing.

So Shoob and Shipley were told to screen the mail elsewhere. "We got moved to an old gun factory in the Navy Yard," says Shoob. "We weren't supposed to get near any military personnel because we were told they weren't insured if something should happen opening the mail. Then I thought, 'Hey, we're not insured either.' Two days later Shoob and Shipley and 30 sacks of backed-up mail were sprung from the arsenal when the White House stepped in and said it would do the screening.

Now Shoob and Shipley are driving a gray Ford LTD from the GSA car pool to the EOB to get the mail. At first they were given a Pinto but they had trouble getting all the mail sacks in it. Shipley is driving and Shoob, up from Savannah, Ga., is musing over the statue of General Sherman across from the Treasury Building. "I think one night we ought to replace him with Lee," he says.

Shoob is a close friend of Chip Carter's. They went to Georgia Southwestern College together and Shoob visited often at the governor's mansion in Atlanta, where he got to know and like Chip's father and practically had a room to call is own.

"One morning I'm sleeping late, to about 10 o'clock," he says, "and suddenly the door starts to open. I jump out of bed and there I am, beard, long hair, I'm in my boxer shorts, and this ladies' tour is coming through. Right through the bedroom."

Shoob and Shipley bring the mail back to Transition City.

"Mush, mush, woof, woof," says Shipley, dragging in the bags.

Lloyd Corwin is in charge of the mail room. Last June he got a PhD in English and American literature from Brandeis. After finishing off his dissertation on Faulkner, he couldn't find a writing job he liked and joined the Carter forces. They gave him the mail room.

Everybody agrees he's done a splendid job. "I shouldn't believe they could catch up with all the mail," says John O'Toole, a George Washington University student seated in front of a bank of pigeonholes and opening "People, Box 2600" mail.

"People is the newest of five categories of mail and, now that people are learning about the "People" address, is expected shortly to cut into the "Jimmy mail," which is anything addressed to the President-elect and is categorized in 14 sections of pigeonholes. There have been almost 200,000 pieces of "Jimmy mail," and Shoob says, "We knew the South was rising again because slavery was alive in the mail room."

The other categories are labeled "TIP" (the so called Carter "talent inventory program," which gets resumes), "VIP" (if a name is recognized the letter avoids the mounds of "Jimmy" and "People" mail and lands in the smaller "VIP" pile), and, the last category, "staff."

Even though every staff member has a pigeonhole, - staff pigeonholes are color-coded - "staff" mail can sometimes be vexing because some volunteers at Carter headquarters tend to come and go, but one can never be quite sure they're really gone.

"You never know when somebody will come in and say, "Where's my mail,'" says Allen Miller of Atlanta, who is affixing return-to-sender notations on two traysful of mail addressed to "staff" members nobody ever heard of and wondering if one or two of these mysterious addressees might materialize when it's too late.

Other letters that don't get past the Shoob-Shipley mail sack-dumping room are those containing threats to Carter, which go into a box marked "U.S. Secret Service."

The "People" mail moves on from the mail room readers to three higher level transition workers, who are just beginning to read it that afternoon when a still higher level worker asks for a "report", by evening on what "The People" are recommending.

The official word is that Jimmy Carter is going to read some of these letters for himself, soon.