One morning, Ann Williams's mail carrier rang the doorbell of her Denver home. "I just wanted to see what you look like," she said when Williams opened the front door. "You're the only person in the neighborhood who gets real mail -- letters."

While everybody else's mailbox is filled with junk mail -- fund-raising appeals, catalogues, sweepstakes, advertisements, bills, and dispatches to "Dear Friend" or "Dear Occupant" -- Ann Williams's mailbox is filled with personal letters. It has been ever since she discovered The Letter Exchange, an organization dedicated to reviving the epistolary art -- that fine and nearly forgotten commerce of personal correspondence.

All it took was a small ad she placed in The Letter Exchange magazine: "What goes on in your head while you're doing the dishes, mowing the lawn, driving the freeway?" That simple invitation elicited 40 letters from total strangers wanting to share those little gems of thought that pass most people by like white lines on the freeway. At one point, Williams was writing nearly a response a day. But she insists, "It's a joy, not an obligation."

The Letter Exchange exists for those who, like Williams, a 50-year-old mystery writer, think of letter-writing as a joy. It is for those arcane souls who would rather reach out and touch someone through a letter than through a telephone, computer or fax machine, for those who understand the simple fact that if you don't write letters, you don't get any.

The brainchild of Stephen Sikora, an Albany, Calif., carpenter who never quite nailed down a doctorate in the history of literacy, the exchange started eight years ago as an ad in the back of Harpers magazine to help his great-grandmother find other people who shared her love of the book "The Tao of Physics." The result was a letter-writers' collective.

Known as TLE, the exchange is a quarterly directory-by-subscription ($12/year) with 3,000 subscribers and 400 ad listings -- sort of personals ads for the literary set. Those listings generate more than 6,000 letters -- and typically any one of them might attract as many as 100 responses.

Sikora says TLE, the only such clearinghouse in the country, differs from pen-pal clubs in that it is "concentrated more on the pen than the pal, more like literature."

Here's how it works:

You find a listing that strikes your fancy and write a letter to that person, who is identified by a code number. You put that letter, with the code number written on the front of its envelope, inside another envelope, and mail it to The Letter Exchange (P.O. Box 6218, Albany, Calif. 94706). Then, Sikor and staff (his family) re-address the letter and ship it off. The rest is up to you.

TLE also offers the opportunity to write to prisoners, to join letter-writing round robins and to connect with magazines that rely on the writing of its readers. It also has sections for women only, for children and for international letter-writers.

In any given issue, the range of the letter-writers is expansive, too. Some ads are to the point: "Lao Tzu, anyone?" or "Let's talk Teddy Bears!" Others are oblique: "Fury, joie de vivre. She's feet firm, head starward, seeks such tall persons in her post (jazzy/profound), Nature, P.H.C., classical, state O'world, cats, teach her, your story."

Some are highbrow: "The implications of the sigmoid curve of Jonas Salk, Jaynes' 'Origins of Consciousness in the Breakdown of the Bicameral Mind,' and mushrooms as sexual form interest and fascinate me." Others are homey: "My entressed are histiry and mans unhumanity to hemself. The wenter mounths keep me inside a loot and ive allwase loved letters, and i beleav i can share som of lifes ancers."

Humor has its letterhead here, too: "I'm not looking for love and marriage or a trip to the Hot Pillow Motel, but if Omar Sharif writes to me you can forget everything I just said." So does somber: "I always feel lonely, bored, depressed, anxious, tired. Anyone out there found a cure?"

The ads in the Kids Corner run the same gamut. Eight-year-old Andy wrote: "I like guitar, bowling, math and collecting money." From 10-year-old Pat: "Farm boy can get out of shoveling manure if there are letters to answer. Please write immediately."

Most people, says Sikora, think of letters as "something they sent to Grandma, thanking her for a lovely pen set she gave them for Christmas. It was formal, predictable, you knew who was on the other end, and anyway, Mother told you to do it.

"Writing for strangers, though, is like writing a novel. You have an imaginary audience, an ideal reader, and it helps you shape your words, helps you find the voice that is yours alone. But unlike writing a novel, it's as easy as talking."

Maybe easier. Since you are anonymous, writing letters to strangers shares a certain commonality with talking to strangers on airplanes: Intimacy goes into warp-drive. You tell them things you wouldn't tell your father-confessor. Says Exchanger Larry Allis, a Pennsylvania engineer, "There's no reason not to be honest with how you feel about your brother-in-law, because it won't get back to him."

In a sense, TLE provides people with a remedy for two seemingly irreconcilable desires. It offers just enough anonymity to release in people the freedom of expression, while allowing just enough intimacy to counteract the very anonymity. "People are hungry for personal address," says Sikora, who personally addresses TLE letters 10 to 12 hours a day. "The Exchange is an antidote to the theme of most of modern literature: alienation. I think the most important part of a letter, for people, is the part right after the word 'Dear ... ' "

Anona Kam, a San Jose occupational therapist, prefers to keep her letter-writing relationships strictly postal. "One thing I like about The Letter Exchange is that you get all the joy of friendship, with none of the obligation," she says. "I don't want to meet my fellow letter writers. I already have friends and family. I want to share communication. I don't want them in my house. I don't want to have to feed them."

Sikora believes TLE is helping to democratize literature. "It brings back a vital connection between writer and reader," he says, "and helps personal expression flow as easily as the words you exchange across the back fence or over shopping carts."

But it takes two to tango. "Plato once said that the life of the mind requires two people, and I think that goes for writing, too," says Sikora. "I can't write unless I have someone to write to ... I need the spark of that other mind to make it go."

And the beauty of letters, adds Anona Kam, is that, unlike conversation, "there is no one to interrupt you."