FORTUNE — For about a decade, people outside of the conservative movement (and some within it) have been mystified by the transformation of American conservatism. How did it become so radical? How did the very idea of government, as opposed to just government excess and high taxes, come to be regarded as evil? And the biggest mystery of all: Why have so many people in the middle and working classes signed on to a political worldview that explicitly puts the interests of the rich and of corporations ahead of their own?

Gary Weiss’s answer: “The credit, or blame, lies squarely with Ayn Rand.” This is an overstatement. Much more credit, or blame, lies with Roger Ailes and Rush Limbaugh. But in Weiss’s new book, “Ayn Rand Nation: The Hidden Struggle for America’s Soul,” the longtime business journalist makes a solid case that Rand — the anti-government author of The Fountainhead, Atlas Shrugged, and The Virtue of Selfishness — is, if not solely responsible for the radicalization of the right-wing, certainly a major influence behind it. Weiss’s warning is that, as ridiculous as many of us might believe Rand to be, we’d better take her seriously.

Rand is more popular now than she ever was during her lifetime, when she was considered a fringe cult figure and often written off as a weak thinker. Weiss never quite explains how her newfound popularity came about. And, he never proves that Rand is the main reason for the Tea Party movement or the rightward shift in general. She has been latched onto by a growing group of people who were ready to accept her philosophy. But it’s easy to imagine the Tea Party emerging even if Rand had never written a word. Her popularity comes thanks to the rise of the radical right, not the other way around.

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Still, it seems likely that the movement wouldn’t be quite so irrational if not for Rand’s influence, and Weiss deftly explains how — despite her atheism — large swaths of the American right have become enamored of her. As he notes, a fair number of Tea Partiers and their compatriots have read Rand, and many are big fans, carrying John Galt — a character in Atlas Shrugged — signs at rallies. Her books are selling like cupcakes. CNBC reporter Rick Santelli’s February 2009 on-air rant against the “losers” who were facing foreclosure is widely credited with launching the Tea
Party movement. Santelli is a Randian. Limbaugh often mentions her favorably, as does Glenn Beck, though neither is strictly a Randian.

And perhaps most importantly, one of Rand’s closest followers and chief lieutenants for decades was Alan Greenspan who, though he hardly mentioned her later in his life, certainly worked hard as Fed chairman to deregulate the finance sector with Randian undertones. Weiss brilliantly and with mordant humor describes how he did this while convincing Congress and much of the public that he was a sage. After the market meltdown, his utterance before a congressional panel that there had been a “flaw” in his beliefs about the trustworthiness of bankers was taken as a mea culpa. As Weiss proves, it was nothing of the sort.

The best part of the book comes in the final chapter and the epilogue, where Weiss lays out his views of Rand and talks with director Oliver Stone about his aborted plans to make a movie out of The Fountainhead. (King Vidor made a famous version of the film in 1949, starring Gary Cooper and Patricia Neal.) Refreshingly, neither Weiss nor Stone are reflexive Rand-haters, and both of them recognize that there was some literary merit in her books, and even some nuggets of wisdom to be found in them.

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Rand was no great artist, and her books were deeply flawed. They were filled with long speeches that no person would, or could, ever actually give. Her characters were bloodless, wooden, and impossible to believe, and her situations and settings were beyond unlikely — — a fatal defect for any story that hopes to describe how the world should work. Her prose, though not unreadable, often landed with a thud. But the plots were engaging enough, and other than the speeches, they were paced pretty well, especially given their length.

As Weiss notes, the appeal of The Fountainhead, the less political of the two books, should be no mystery to anyone with an independent streak. It’s telling that, as Weiss notes, Howard Roark, the main character, didn’t care about money, but only about his work as an architect, which he insisted must always be done his way, by him alone. The later Atlas Shrugged had a far more pecuniary message, setting up the capitalist (in those days, the industrial capitalist) as the ultimate hero, the best thing a person could possibly be. Both books “are beguiling,” Weiss writes, “as they conflate self-esteem with self-indulgence, enlightened self-interest with selfishness, discarding the ethical concepts that make life bearable.”

The lack of such ethics, of course, is what ultimately makes both books “staggeringly immoral,” Weiss concludes. This despite the insistence of Rand and her followers that she was all about ethics — the ethics of selfishness, which she said were the only ethics that mattered. She despised altruism. The uninitiated often believe that there must be more to Rand’s philosophy of “Objectivism” that would reveal it to be less harsh and, well, loopy than what it sounds like. But there isn’t.

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Weiss interviews several Rand fans of various levels of fervor (and of various levels of understanding: some of the most entertaining parts of this entertaining book are where Rand adherents betray their utter ignorance about basic economic and political concepts, Rand herself, and even their own strongly held beliefs). Nearly all of them came to Rand, as people generally do, at young, impressionable ages, when they were most vulnerable to simplistic — or simple-minded — ideas. Most of them here seem to be driven far more by emotion than by reason, though of course that’s a problem for ideologues of all stripes, left and right, as a quick look at any political comments section will reveal.

The ultimate flaw of Randianism is the ultimate flaw of any ideology that is designed to be pursued myopically and for its own sake, more as a fundamentalist religion than as a social philosophy: there’s no flexibility built into it. That was Communism’s fatal flaw, too. Such ideologies utterly ignore the way real people think and behave, and what real people value, and instead pursue impossible dreams, which naturally leads to reason being trumped by emotion. Everything becomes zero-sum. It’s the individual vs. the tribe, or the collective (as represented by the state). We can’t live in a world where the individual always rules because the actions of individuals always affect other individuals. And we can’t live in a world where the tribe always rules because that would sap life of its joy, and nothing would get done. Real life is lived in the tension between those two poles. We pursue our rational self-interests while also looking out for others. It’s messy, but the messiness of public life is what democracy was invented to reflect and address.

Rand and her followers have pursued “an ideology that appeals to American values, but is simply not American,” Weiss writes. As he makes clear, what we need in government, now more than ever, is an understanding of how life is really lived, and Ayn Rand is just about the last person we should expect to guide us toward that reality.