"Until now the Queen had met Kate only fleetingly at the wedding of Peter and Autumn Phillips, but she went out of her way to make her feel welcome, giving her permission to take pictures at Balmoral," Nicholl wrote in her book.
"As a woman who has lived her entire life in the public eye, the Queen rarely lets her guard down, and very few apart from her family and closest friends get to see the real Elizabeth," she added.
Prince William Kate went on to wed two years later in a lavish ceremony.
WATCH: MEGHAN MARKLE COMPLAINS ABOUT BEING A ROYAL