Gywneth, who tends to keep her children out of the public eye, was accompanied by her daughter Apple, nine, and son Moses, seven, who both bear a striking resemblance to their famous mother.
The It's All Good author previously revealed that she was inspired to write the book after a health scare and doctors' orders to follow a strict diet, and that she raises her children on a gluten-free diet.
"What it is," said Gwyneth, "is that my son has very bad eczema and he's allergic to gluten, and my daughter's allergic to cow dairy. I try at home to make everything gluten-free for him because the difference in his comfort is unbelievable when he's sticking to what he's meant to be eating."
She also laughed off criticism about their regimented diet, saying, "They say I don't give my kids carbs, and I'm feeding them seaweed — I have no idea. But they eat Oreos, too, and they're normal kids."
Gwyneth also took the opportunity to chat and mingle with other celebrities, including fundraiser Alec Baldwin and his pregnant wife, Hilaria Baldwin, at the book-signing event.
Dressed in an immaculate white dress, Gwyneth looked her toned, tanned and healthy self and has no doubt been enjoying the summer sun eastside. She recently relocated her family back to the US after having lived in London, in time for her children to start the next academic year.
"Gwyneth has loved living in London but America will always be home and she wants to be closer to her family," said an insider. "She misses her family terribly and now is the time to move.
"Her children have English accents and once they start senior school Gwyneth says the family will likely return to the UK because she believes that the English education system is second to none."
Gwyneth and her husband, the British-born Coldplay frontman Chris Martin, are likely to move into the £6.6million home they own in the Hollywood Hills.
The house, which is located in the exclusive Brentwood area, is just one of a number of properties own by the couple.
The famous couple also reportedly have a £4.6million home in North London, a £3.2million penthouse apartment in New York and a £3.4million house in the Hamptons.