A NEW party-planning book written by Prince William’s shapely sister-in-law has turned out to be a major royal embarrassment!

Pippa, younger sister of royal bride Kate Middleton, is being accused of cashing in on her royal connections after being paid a whopping $600,000 advance for the how-to-guide party book, “Celebrate: A Year of Festivities for Families and Friends” – and critics charge it is so simplistic it could have been written by a child.

“Kate and Prince William must be mortified over the book,” said a source. “If Pippa had any decency at all she’d hand the money back to the publishers.”

Pippa’s far less than insightful tips include: “When hosting a party, move any clutter from the space where you’re entertaining” and “Nearly any breakfast becomes special when it’s brought to you in bed.”

Said the source: “It’s a guide that’s geared more toward a third-grader than the adult public, and it’s quickly become the biggest joke in the book world.”

 A reviewer from the “Irish Sun” newspaper quipped: “It is clear that by writing the book, Pippa set out to prove that there is far more to her than her pert bottom. But the fact is, there isn’t.”

Other critics, both in the States and overseas, have been equally harsh, with one remarking that the book is “perfect for anyone who needs a recipe for making ice.”

Pippa, 29, skyrocketed to fame in April 2011 when her shapely butt took center stage at Kate and William’s wedding. Since then, her wild ways have drawn the ire of the royal family on more than one occasion.

According to the source, the palace objected to Pippa’s writing project, but she went ahead with it anyway.

“Now she’s getting raked over the coals for it,” said the source. “It’s extremely demoralizing for Pippa, her family and the royals.”

Pippa tried to defend herself, but it turns out her only experience at entertaining comes from working as a kitchen assistant at a British pub.

To save herself further embarrassment, Pippa canceled a scheduled U.S. media tour in support of the book, even turning down interview offers from Oprah Winfrey and Anderson Cooper.

“She’s decided to lay low in London until the backlash dies down,” added the source. “And if reviewers and wildly disappointed readers continue the bashing, that could take years.”