The mother of a young cancer sufferer sent her heartfelt thanks to Harry Potter author JK Rowling for providing comfort as her daughter underwent treatment.
The moving letter was read by actress Ellie Bamber, who was joined by Sir Ian McKellen and Jude Law for the latest star-studded performance of Letters Live, a series of performances in which remarkable letters are read aloud.
Chrissy Hart wrote the letter to the author last month, describing how the Harry Potter series had provided comfort to her "beautiful daughter" after she was diagnosed with cancer.
The young girl sadly died this week, Letters Live organisers said.
She wrote: "Your words built a castle for her to move into when the prognosis got worse.
"Mrs Rowling, cancer threatened to take everything from my daughter, and your books turned out to be the fortress we so desperately needed to hide in."
Mrs Hart added "the most important lesson you have taught us" was "for every child that is taken too early, people will come together, huddling close in a big group, wands held high to light the dark sky with the love they felt".
The letter concludes: "You built Hogwarts as a home for her to return to. And for me."
Author Julia Donaldson, Sanjeev Bhaskar and former X Factor presenter Caroline Flack also performed at the matinee event at the Freemasons' Hall in London.
Letters by Roald Dahl to a girl who was not enjoying school, actress Elizabeth Taylor to her missing cat and American president Barack Obama to the daughter of a gay couple were read to the packed audience.
The event is based on the collection of letters published in Shaun Usher's international best-seller Letters Of Note.
The 2016 series will run until Tuesday and feature dozens of other famous performers, including Benedict Cumberbatch, Olivia Colman and Carey Mulligan.
Letters Live first launched last year and quickly became a sell-out success.