The younger sister of Kate impressed in a Stella McCartney dress at the event held at Daunt Books in Chelsea.
She revealed that she was "really, really, nervous" for her big day.
As well as her Stella dress, the 29-year-old was spotted wearing an £815 dress by Roksanda Ilincic, a £395 multi-coloured tunic dress by Paper London and a £255 Markus Lupfer green-and-black bodice dress.
Inside the book, which was released officially today, Pippa writes: "I know many of you will pick up the book out of nothing more than curiosity.
"But I can assure you that it feels even stranger to me than it probably does to you to have seen so much written about me when I have done so little to paint a picture of myself.
"This is my first chance to do that and I've enjoyed every minute of it."
In the introduction to her book, Celebrate, Middleton acknowledges that "it's a bit startling to achieve global recognition (if that's the right word) before the age of 30, on account of your sister, your brother-in-law and your bottom".
Last night, she said: "I feel incredibly lucky to be in this position to publish a book. It's been a crazy few years since my sister's wedding.
"It's had its upsides and its downsides and I feel really fortunate to build a career as a writer."