Moving from Missoula, Mont., to Brooklyn, N.Y., is not the longest journey Frances Kuffel has ever taken. At the age of 42, the literary agent/writer/poet jumped into a 12-step program that took her from 313 pounds to 125 pounds in just 21 months.

She ate mostly fruit and chopped vegetables. Then, quite suddenly, Kuffel's new five-foot-eight-inch frame became "a stranger in the mirror."

Her book, "Passing for Thin: Losing Half My Weight and Finding My Self," is not a diet book — even though some bookstores mistakenly display it as such. It is the alternately sad and delightful tale of a woman who went from fat to thin so quickly that she experienced both excitement and pain.

Kuffel spoke candidly about "the aftermath of weight loss" during a telephone interview from Santa Monica, Calif. The original motivation for the book's title was "Black Like Me," the story of a white man who passed as black and wrote about it. "I said, 'I could write such a book because I'm always just passing for thin' — no one knows what is really inside me. Then the subtitle was supposed to be 'An Alien Sister to the Planet of Girls,' but the marketing people stepped in and changed that.

"No one can prepare you for suddenly being in this new body. I just thought everything would be fine — and it wasn't! Dieting is easy compared to what comes afterwards. So I've written a cautionary tale. My first draft was angry, suggesting how mad I was at the world. But one of my best assets is my sense of humor, so I used irony to tell the story. I've always been a funny writer. That's who I am."

Kuffel had no idea what it was like to be the subject of "catcalls" from men. "It took me a long time to understand I was being evaluated for my looks, and only later on for other stuff."

She has dated two men seriously since her weight loss: "the Catholic boy and the boy from Connecticut." Both were initially attracted to her because of her looks, then, gradually, they started to appreciate her ideas, "the stuff of me."

Oddest of all, Kuffel discovered the reality of chemistry between a man and a woman. "I never knew about it when I was fat — but it either clicks on or it clicks off. I find that men are not as conscious of body type as I had thought they were."

Whereas she used to wait a bit before telling a man the story of her weight loss, her book has made her better known. "Now they can look me up on Google — and I've had men do that. One thing I found out is that being desperately in love is desperate. When I thought I was in love, I was always incredibly happy or miserable. I was so frightened all the time."

Anyway, she has a dog now. "That is the main thing. I'm even working on a book proposal about my Labrador retriever. It will be called 'Walking Miss Daisy.' She has changed my life in every conceivable way. I love her and I hate her. I wish I had a husband because he could do half the walks. Right now I'm in the middle of canine garbage!"

Kuffel is not interested in being "the poster girl for weight loss, but I can tell you I don't cross my legs without saying a little prayer of thanks."

Losing so much weight has left her with some excess skin in various places on her body, an annoying factor when she buys clothes. "I always have to find a jacket for that sleeveless dress." But she is dead set against having abdominal surgery to carve away the excess skin — "even to make me beautiful."

Having had abdominal surgery twice — once to remove a 36-pound ovarian cyst — Kuffel "would rather die than go through it again. Waking up again under a general anesthetic — No! I know one woman who had surgery for a tummy tuck and now she has no sensation in her lower belly."

Today, Kuffel is in great health and enjoys low blood pressure and low cholesterol levels. Her feet used to hurt a lot when she was heavy — now she gets "horrible corns" on the bottom of her feet. "One toe is too long and the other is too short, so periodically I have to have everything shaved off. I feel like I'm walking on a lit cigarette."

Could she ever put it all back on? For sure. "The temptation is always there. It's like alcohol. No one can succeed at this until they realize their weight is a symbol of compulsion — and that attacking the compulsion is what they must do. I did a 12-step program because I needed a lot of support."

One of her biggest motivations came from the slightly overweight Janene Garofalo in the 1996 film "The Truth about Cats and Dogs." Garofalo stars as Abby Barnes, a radio host/veterinarian who agrees to meet "Brian from Venice." But then she asks her neighbor, an ultra-thin model (Uma Thurman), to pretend to be her for the date. Kuffel said she liked Garofalo in the film because she is "smart and funny."

She added that "if Abby could enchant someone, maybe I could, too. Maybe there is something to me besides my weight."

Ironically, Kuffel's voice is very similar to Garofalo's.

Given her bent for humor, Kuffel looks at her story as "poignant, even sentimental."

The book is currently climbing the best-seller list. "I'm seeing it change people's lives a little bit — and that's amazing to me. A woman who works in a Tasty Cakes factory wrote me that she started a diet 10 days before Christmas, and already she has lost one dress size."