"Lean In" is a terrible title for her book. It's as weakly euphemistic as "reach out," the touchy-feely synonym for "ask" that sounds urgent only when belted out by the Four Tops. "Lean In" also signals an effort at branding from a woman whose proselytizing is ready for a campaign trail. And it's too dainty to convey what Ms. Sandberg really means: Stand up. Step forward. Speak out. Be smart and strong, and don't torpedo your own efforts in the workplace. That's the assertiveness for which "Lean In" is a landmark manifesto.

Writing this book was gutsy, especially for someone who was prophetic about how it would be received. "Everyone loves a fight — and they really love a catfight," she writes. "The media will report endlessly about women attacking other women, which distracts from the real issues. When arguments turn into 'she said/she said,' we all lose." The pre-publication flap over "Lean In" fits exactly those specifications.

Should some of Ms. Sandberg's most outraged critics care to know what her book actually says, it's an expansion on some of her public statements. She has used a TED talk, a Barnard College commencement speech and a 2011 appearance at Harvard Business School to sound an alarm. At Harvard she bluntly told the audience, "If current trends continue, 15 years from today about one-third of the women in this audience will be working full time, and almost all of you will be working for the guy you are sitting next to."

"Lean In" offers many examples of how women undercut their career potential. In a book that is sometimes personal but should have been much more so, Ms. Sandberg describes her own way of handling that Facebook job offer: She was willing to accept whatever terms its chief executive, Mark Zuckerberg, proposed, until her brother-in-law told her that no man would ever take the first deal.

Ms. Sandberg has led such a charmed professional life that Mr. Zuckerberg did not allow her to refuse him; he made a better offer. And her extremely accommodating husband, Dave Goldberg, gave up his job in Los Angeles so that his wife could further her career in Menlo Park, Calif. He then became a chief executive in Portland, Ore., before moving the company headquarters to the Bay Area to be close to Ms. Sandberg and fully to share parental responsibility for their children.

How applicable is Ms. Sandberg's life in the stratosphere to those of working women on earth? More so than her book's naysayers expected. She identifies a passivity in women that, for instance, makes them reluctant to boast about their own achievements but happy to praise one another's. She advises against waiting for a mentor, "the professional equivalent of waiting for Prince Charming," and excoriates the way mentoring is misunderstood. "We need to stop telling them, 'Get a mentor and you will excel,' " she says of less experienced women. "Instead we need to tell them, 'Excel and you will get a mentor.' " Like a lot of what "Lean In" says, this thought is common-sensical but too infrequently heard.

It would be easier to benefit from Ms. Sandberg's advice if she made herself fully known to her readers. But at the personal level "Lean In" is a mass of contradictions. She describes herself as fearful and tearful, "the kid who got picked last in gym" and the one who had her "most likely to succeed" title yanked out of her yearbook, thinking it would make her unpopular. She describes moments of knee-rattling self-doubt. She cites data indicating that men ascribe their success to drive and ability, while women are apt to mention luck, hard work and help. In a book that has its share of specious research citations she also notes that men are more apt than women to drown.

But there is a Sheryl Sandberg who isn't so meek. And that Sheryl is seen here only fleetingly. Yes, she admits, as a kid she used to make her younger siblings listen to her speeches and then scream, "Right!" in conclusion. There's a Sheryl who wields great authority over large numbers of employees, who risks being overbearing at meetings ("In order for me to speak the right amount in a meeting, I have to feel as if I am saying very little"), who has "a tendency to get impatient about unresolved situations" and who runs a tight ship. But what exactly does this C.O.O. do? She says little about her Facebook role and makes only a glancing, barbed reference to Sergey Brin and Larry Page, the Google founders.

But "Lean In" will be an influential book. It will open the eyes of women who grew up thinking that feminism was ancient history, who recoil at the word but walk heedlessly through the doors it opened. And it will encourage those women to persevere in their professional lives, even if Ms. Sandberg's own domestic and career balance sounds like something out of a fairy tale.

"I am fully aware that most women are not focused on changing social norms for the next generation but simply trying to get through each day," she writes. No, most women do not have Ms. Sandberg's choices. But the most important choice she made with "Lean In" was deciding to write it. Although its author lives a life of great privilege, "Lean In" treats speaking out as the greatest luxury of all.