HarperStudio has signed a seven-figure, 10-book deal with Gary Vaynerchuk, a 33-year-old Belarusian-born wine retailer from New Jersey, who, except for a talk show appearance here and there, is basically unknown in mainstream media circles.

But in the world of the Internet, he is a Twitter phenom, with 145,000 followers hanging on his every tweet. What began as a daily video blog about wine has become a self-help, business-advice juggernaut, with "Garyvee" as chief engineer. As he describes himself online, "I love people,...