I have a good friend who writes popular books for teenagers. She also has nearly 19,000 Twitter followers and recently qualified for a verified account. Consequently, she’s often asked to speak on panels at book conferences to share her obvious expertise in how to use social media to build a loyal audience.

Here’s the thing: she has none. (That’s not a slam, by the way—she’ll tell you that herself.)

What she does have is a very clear sense of who she is, who her readers are, and why they connect with her. She doesn’t spend her time on Twitter hollering about her “brand,” or constantly asking people to buy books she’s already written, or trying to figure out how she should sound so that people will want to buy her next book. She just talks to them, and she responds to them when they talk to her. And then they buy her books, for a couple of clearly observable reasons:

1) The bond she’s forged with her audience is so strong that a number of readers have picked up copies of her books after finding her on Twitter first. She tweets a lot, and all of her tweets share certain core qualities: an offbeat, irreverent (but not snarky) sense of humor; a passionate commitment to equality and fairness; generosity; and a sense of adventure.

2)When they buy her books, readers find that no matter how disparate the subjects, the books share those same qualities. Readers who love those qualities love those books. And they’ve helped put one of them on The New York Times bestseller list.

So if she doesn’t have a degree in marketing or a career as a social media expert (if such creatures can truly exist in what’s still the dawn of the social media age), how did she do it—especially when publishing is an industry in flux? And perhaps more importantly, since you’re reading this blog and not a young adult novel, how is this relevant to branding?

It’s simple: voice.

Voice is the distinctive tone and style in which we communicate. It’s not just words; it’s a fusion of language, design and content. Voice is the external expression of what makes us unique—as people, certainly, but also as companies. Unique is appealing. It’s engaging. It feels personal. It creates a dialogue with the people who consume whatever it is you make, whether it’s books or clothes or telescopes.

It’s usually more difficult for a company to identify its true voice than it is for an individual (raise your hand if you’ve ever sat in a meeting where powerful ideas were pulverized into nonsense with the sledgehammer of groupthink), but it’s even more critical.

Because voice isn’t a veneer. It can’t be invented or imposed without sounding instantly, irredeemably false. True voice is an authentic expression of your core values—and that means that it’s also a perfectly calibrated tool for measuring how well you’re embodying those values. (Notice all the talk in the blogosphere and elsewhere about the cognitive dissonance generated by the Google-Verizon deal, because it sounds at odds with Google’s informal “Don’t be evil” motto? That motto was a really clear example of voice in action. Obviously, it still is.)

Whether you’re self-employed or run a multinational corporation, a clear, consistent, authentic voice is one of the most powerful tools available for attracting and retaining an audience. It can take some digging to uncover your voice, and that voice can make demands on your behavior once you do. But the rewards of being able to express exactly who you are and why you do what you do—not to mention having clear criteria for how well you’re fulfilling your mission—are rich.

Julie Polk is a writer for the Siegel+Gale New York Office.