What keeps people pining for Pippa Middleton?

• Often seen but seldom heard, Duchess Kate's little sister intrigues all on her own
• Middleton's new book, 'Celebrate,' offers window into a near-royal's world
• She's 'half princess, half commoner -- the accessible royal'

Always a bridesmaid, never a bride — and that's how the public likes her.

Pippa Middleton is "the world's most eligible single woman," as Daily Mail contributing editor Catherine Ostler puts it. "Her Royal Hotness" is also easy on the eyes — and ears. Since the world was introduced to her (and her enviable backside) at sister Kate's wedding to Prince William, Pippa is widely seen — in her smart shifts and smoky sunglasses — but she seldom speaks.

All of which adds up to one alluring woman.

"She seems half princess, half commoner — the accessible royal," says Ostler, the former editor of British society magazine Tatler. Indeed, her fame eclipses that of real royalty, Princesses Beatrice and Eugenie. Middleton was the second-most-popular woman (behind her sister) to have her photo hit the front pages of British newspapers this spring, according to a study by the United Kingdom's Women in Journalism organization.

The younger Middleton, 29, is opening the window of access that much wider with the Tuesday publication of her widely anticipated book, Celebrate: A Year of Festivities for Families and Friends, in which the world gets a sense of her, well, cheeky voice.

"It is a bit startling to achieve global recognition before the age of 30 on account of your sister, your brother-in-law and your bottom," the party planner writes. "I certainly have opportunities many can only dream of — but in most ways I'm a typical girl in her 20s trying to forge a career and represent herself in what can sometimes seem rather strange circumstances."

The tone ratchets up her charm, Pippa watchers say. "She sounds down-to-earth, quite self-aware and rather romantic and optimistic in a way — though dreaming up 'fun' parties is quite a misty-eyed thing to do, anyway — schmaltzy even," Ostler says.

In a way, Pippa Middleton is Lee Radziwill to Kate's Jackie Kennedy Onassis. The party planner (Middleton) and the interior decorator (Radziwill) are each "glamorous, with an interesting love life," Ostler says. "And there is something so retro about the (Middleton) sisters: their style, their aspirations, their hair."

"She gets some of the benefits of being a (royal) sister-in-law without the full-on responsibility and scrutiny," says People's London correspondent, Simon Perry. As in, "do I really want the responsibility and heartache and headache that go with being the queen someday?" For that reason, her ability to put one foot into her sister's Kensington Palace pad and keep the other in her Chelsea flat, "she's someone to aspire to."

That said, as a de facto duchess, Pippa Middleton has had to make some sacrifices, from her dress to her demeanor. "She is in a unique position," says Simon Doonan, Barneys New York's British-born creative ambassador and the author of Gay Men Don't Get Fat. "The sister of a future monarch must toe the line and make discreet choices. She cannot embarrass her sister with excessive partying or displays of cleavage or drunkenness. The British public would be quick to turn on Kate and Pippa if they started behaving in a self-indulgent, chaotic way. Theirs is essentially a life of service. This is why she dresses conservatively and keeps her trap shut.

"There is an additional burden on Kate and Pippa," Doonan adds. "They must be above reproach because they are 'commoners.' They must prove that non-aristocrats have just as much class as toffs (snobs), if not more."

And aside from a couple hiccups involving a French aristocrat friend brandishing a fake gun and a vaguely S&M-themed costume party, the younger Middleton has comported herself with grace. "All the Middletons have taken their instant fame and coped with it very well," Perry says. "There hasn't been a snarl, they don't show anger to anyone." Even with paparazzi a persistent presence, Pippa Middleton "just keeps calm and has a good way about her."

And as long as her sister is a royal, interest in Pippa will be piqued — "though it will probably diminish once she has chosen/found a husband," Ostler says.

Unless, of course, that husband happens to be the world's most eligible bachelor — her sister's brother-in-law, Harry.