CHICAGO - Poet Mark Strand calls dust-jacket photos "the door to the dark room of the imagination."

Even though a writer's looks have little to do with the words he or she puts down on paper, many a reader, during the course of a gripping or particularly insightful book, will turn to the author's photo, trying to plumb its mysteries.

"When you're really sucked into a book, it's an incredibly intimate experience," says Justine Larbalestier, an Australian science-fiction writer and historian. "You're really in that person's brain."

As an author, Larbalestier is ambivalent about dust-jacket photos. She's even tried, unsuccessfully, to persuade her publishers to go without her photo. But, as a reader, there are times she finds herself gazing at the writer's image for answers. She did so recently while reading "The Music of the Primes: Searching to Solve the Greatest Mystery in Mathematics" by Marcus du Sautoy.

"It's about math, but the writing is passionate and beautiful," Larbalestier says, "and I kept trying to reconcile the smug picture of the author with the elegance of the text."

The evocativeness of dust-jacket photos is why publishers put them on the cover. They're selling tools, part of the book's packaging, like the packaging on a bar of soap. Yet in the work of some photographers, the author's photo can aspire to the level of high art.

Among the foremost photographers to specialize in this sub-genre are Nancy Crampton, who recently published a collection of her work, "Writers" (Quantuck Lane Press, 224 pages, $40), and Jill Krementz, whose book of author portraits, "The Writer's Desk," was issued by Random House in 1996.

A third is Marion Ettlinger whose own collection, "Author Photo," was put out two years ago by Simon & Schuster. Known for her distinctive black-and-white portraits which have the flavor of old-time movie stills, Ettlinger - who abhors having her own picture taken - has photographed more than 600 writers since 1983. She's aware of the incongruities of the process.

"They're being photographed because of their intellect, their inner life, and here they have to do these things involving their appearance. It seems like a big contradiction," she says.

Before photographing an author, Ettlinger tries to read the writer's new book. "When I have read the book, I feel I've just been inside the head of this person," she says. "I want to be in harmony with the tone of the book. If it's a book about a tortuous subject with a lot of heavy emotion, I don't want the writer to look casual or gleeful."

Actually, Ettlinger rarely allows her subjects even the barest glimmer of a smile. "I'm interested in a sense of timelessness," she says. "I want the viewer of the photo to feel the presence of this person."

Publishers bank on that connection between the reader and the author through the dust-jacket photo to help sell the product. Knopf includes an author photo on virtually every book. Most of them are commissioned at a cost ranging from of $300 to $5,000 plus the photographer's expenses, says publicity director Nicholas Latimer.

The dust-jacket photo is part of the selling package that includes the cover art, the background color, the author's name, the book's title, the author's bio and the back-cover blurbs - each trying in some way to hook a potential book-buyer's attention and interest.

When Latimer sends someone to photograph an author, such as comic mystery writer Carl Hiaasen, he says, "I want a studio shot where he looks like a writer, something that can be cropped so it can be vertical or horizontal. I may know the book jacket is going to be orange, so I'll say, 'Don't let him wear an orange shirt."'

Often, big-name writers will tell the publisher whom they want to take their photograph. "John Updike's wife has taken a lot of his photos," Latimer says. "She's a retired social worker. He tends to like to go with her images. They're not great, but he is comfortable in front of her, so you get something out of it."

The publicity value of the dust-jacket photo extends beyond the book itself. The author's mug becomes an essential element in print advertising and other promotions, such as bookstore appearances. In addition, there's always the hope, Latimer says, that, if the photo is interesting enough, it can help the book get coverage in the glossy magazines or on the pages of newspaper book review and feature sections.

That's why Latimer sent Elena Seibert out to northern California to photograph author Jane Smiley sitting in the midst of the 101 novels she read and analyzes in "Thirteen Ways of Looking at the Novel," which Knopf published in September. And that's why the color photograph takes up much of the back cover.

The fear at Knopf was that magazine and book review editors might take a pass on Smiley's book since it's non-fiction and Smiley is best known as a novelist. "A good photo of her might sway (the decision) in the other direction," says Latimer.

Smiley, who wrote the best-seller "A Thousand Acres," is no fan of dust-jacket photos. "My first book-jacket photo - I was 20, and (the photographer) made me look 45. I look prematurely embittered and mean," she says. "I hate having my picture taken."

But she was game for the Seibert photo session and had such a good time that, near the end, she suggested a photograph of her in the bathtub with all the books piled atop her - a shot Knopf has included in publicity materials for "Thirteen Ways of Looking at the Novel."

"It's a gimmick. It's more fun," Smiley says of the two photos. "Usually, you're just standing there, trying to look compelling in some way and not succeeding. It can look like a mug shot. The author can look, not exactly like a deer in the headlights, but a little bit like another night creature scuttling from burrow to burrow."

Novelist John Searles, who is also the book editor at Cosmopolitan magazine, says, "There are times when I flip to the back of a book as a reader or editor, and, when the person (in the author photo) looks a little too dark and brooding, or too posed and self-important, that's a turn-off. You want something inviting."

Although he has written humorously about having his own dust-jacket photos taken, Searles has no problem with his image being treated as marketing material. "I'm of the mind: Use whatever you can to get your book into the hands of readers," he says. But he adds, "I don't think people will buy it because of the photo."

That's the attitude of San Francisco-based Chronicle Books, which puts no premium on dust-jacket photos. "We tend to go with passport photos, more or less," says Jay Schaefer, an editorial director at the publishing house. Or no photo at all.

Rarely will Chronicle commission a photograph. Occasionally, Schaefer or someone in the office will take the writer's picture. In most cases, the author is asked to supply a head shot. For example, the dust-jacket photo of Nadia Gordon, author of "Murder Alfresco," was taken by a friend, and the one of Kristen Sundberg Lunstrum, author of "The Life She's Chosen," was by her husband.

"For us, the photo is the tail of the dog," says Schaefer. The presence or absence of a dust-jacket photo "has no appreciable difference on sales."

Yet, dust-jacket photos, whether postage-stamp size or filling the entire back cover, are evocative because, as the poet Mark Strand notes, they hint at the essential mystery of creation.

But when three of the best known dust-jacket portrait photographers - Ettlinger, Crampton and Krementz - published collections of their work, each handled her own author photo differently.

Ettlinger took the photo herself, even though being on the other side of the lens took a lot of getting used to. "At first, it was kind of a baffling, discombobulating experience," she says. "Who am I looking at? Who am I?" But finally she snapped an acceptable shot.

Crampton says that, for her book, she forgot about an image of herself. "It wasn't a conscious decision," she says. But she adds, "I'm happy being on the other side of the camera."

Krementz came up with a third approach for her book-jacket photo. Author Ursula K. LeGuin snapped the shutter.