Kate Middleton has left copies of her new book with hidden letters across the UK to celebrate its launch.
The Duchess of Cambridge said on Twitter on Friday that she had placed 'Hold Still' at secret sites all over the nation.
The book - which sells for around £20 - has already raced to the top of the Amazon 'Best Sellers' charts.
A video accompanying her tweet showed Kate, 39, placing a copy of the title in the grounds of Kensington Palace.
Posting on her official account, she said: "Let the search begin!
"We’ve joined @the_bookfairies for the day to share copies of Hold Still around the UK with you.
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"Each copy is adorned with a gold book fairy sticker, gold ribbon, and has a letter from The Duchess tucked inside."
A second tweet added: "To make this activity even more special, book fairies, the Hold Still judges and participants of the final 100 images are leaving copies at places that gave them hope during the lockdown.
"This special book documenting the unique collection of photographs goes on sale today!"
Kate seen later on Friday visiting the The Royal London Hospital and the National Portrait Gallery in a surprise public appearance.
Her book - full title 'Hold Still: A Portrait of Our Nation in 2020' - documents the UK during the pandemic.
It features 100 pictures selected from around 31,000 entrants and has been produced in partnership with the National Portrait Gallery.
The Book Fairies - who encourage people to hide books for others to read after finishing - tweeted their happiness at their involvement with the duchess.
A message read: "We are delighted that The Duchess of Cambridge has not only harnessed the power of The Book Fairies today but became a book fairy herself to hide copies of Hold Still!
"Follow #HSBookFairies to follow the activity today!"
People have already posted pictures on social media showing them discovering 'Hold Still' across the UK.
Kate launched the project in May last year and encouraged the public to pick up a camera or smartphone and capture the "spirit of the nation".
Images of sleeping nurses, shielding elderly people and family scenes showing the reality of working from home were some of the images submitted.
Net proceeds raised from the sale of the book will be split between leading mental health charity Mind and the National Portrait Gallery.
