Single women are flocking to buy a little pink book that states what seemingly would be obvious: If he doesn't act like he's into you, it means — he's just not into you!

Written by two former "Sex and the City" writers, Greg Behrendt and Liz Tuccillo, "He's Just Not That Into You: The No-Excuses Truth to Understanding Guys" (search) tells women that if he isn't calling you or treating you like the cream of the crop, then frankly, he doesn't give a damn.

"I have seen love change dogs — women can bring out the best in men. The reality is until he meets 'the one,' he is likely to go astray," Behrendt told FOXNews.com.

The book, which has topped the Barnes & Noble, Amazon.com and New York Times bestseller lists and sold out in many stores, was inspired by a "Sex in the City" (search) episode in which the girls offer Miranda (Cynthia Nixon) multiple theories on why her date turns down a nightcap. Nothing seems to explain it, until the end of the episode, when Berger (the boyfriend of Carrie, Sarah Jessica Parker's character) brutally lays on his male perspective: "He's just not that into you."

The advice is tough to hear, but it sets Miranda free.

Behrendt says he has received some pretty nasty and even threatening e-mails from guys who got the boot on account of this book. But many men say the advice, which claims that men are not complicated at all but too scared and/or lazy to ditch a woman who isn't "the one," hits the nail on the head.

"Yes, sometimes it is just easier to continue to date [women] then to go through the process of breaking up with them," said 29-year-old New Jersey native Rich Viana.

Viana, a senior computer network engineer, has done his share of dating since moving to New York City. And he admits there are times when "my cell battery is dying" or "I have to work late" is a lot easier than "I'm just not that into" talking to you right now.

Behrendt was inspired to write the book after countless nights of trying to tell his little sister that she could do better than the jerk who had her crying herself to sleep. And like a big brother scorned, this book doesn't soften any blows: "Busy is another word for [expletive] and [expletive] is another word for the guy you're dating" is one quotable line. "No answer is your answer" is another.

Co-author Tuccillo has been a victim of Mr. He's Just Not Into You herself. She remembers fighting with her friends, telling them they "just don't understand how important this is to him" after a guy she was dating text-messaged her that he was quitting smoking and needed some space.

But do the authors offer good advice? Relationship therapist Mary Rower (search) said it's a start, but that women finding themselves in situations with uninterested guys should do some soul-searching before doing more male-searching.

"I love the book. I just think that rather than just blow the guy off, it should also make a woman look at herself and see why she would be with someone like that in the first place," she said.

Tuccillo said some fellow "Sex and the City" writers did question her on two lessons from the book: that a woman should leave a man who doesn't want to marry her, and that a woman must never ask a man out.

But she hasn't had a change of heart on any of it.

"At the end of the day, 'can't' and 'don't want to' both mean the same thing. That is that he's just not that into you," she said.

Rower posed the idea that a "She's just not that into you" book may also be needed for men. But Behrendt doesn't think that's likely anytime soon.

"They could write one but it would only sell about eight copies. Generally, guys obsess in different ways than women," he said.