The concept for the book, which is released today, was launched exactly a year ago as part of a collaboration between the National Portrait Gallery and the duchess, who is a patron.
Ahead of the launch, the Kate joined with reading organisation The Book Fairies to hide 150 copies of Hold Still in various locations across the UK.
The Book Fairies is a social initiative which asks readers to hide books they have enjoyed in public spaces for others to find.
In December 2019, Harry Potter star Emma Watson has worked with the organisation to hide 2,000 copies of Louisa May Alcott novel Little Women to mark the release of the film where she played Meg March.
Kate was joined by 73 other book fairies and 46 people featured in Hold Still to hide the books in locations across the country, including on park benches, local landmarks and in transport hubs.
At The Royal London, the duchess learned about its art rehabilitation programme Vital Arts and spoke to a group of hospital staff who worked throughout the pandemic.
She also viewed a portrait called Melanie, March 2020 by Johannah Churchill, which appears on the front cover of Hold Still and will now hang in the hospital.
Kate then visited the National Portrait Gallery’s archive to meet director Nicholas Cullinan, who confirmed all 100 portraits will join the gallery’s permanent collection.
She also met Lotti Sofia, Niaz Maleknia and Claudia Burton, whose photographs feature in the book.
Proceeds from Hold Still will be split between mental health charity Mind and the National Portrait Gallery.
A picture of Mila, who is having chemotherapy for leukaemia, and her father, taken by her mother Lynda, is in the book.
During a telephone call between Kate and Mila, Mila asked the duchess what she was wearing, with Kate admitting she was “not wearing a princess costume right now” but pledged to wear Mila’s favourite colour when they eventually meet in person.
Earlier this week, it was confirmed the Duchess of Sussex has written a children’s book called The Bench inspired by the relationship between her husband Prince Harry and son, Archie.