The star-studded event included readings from Benedict Cumberbatch, Jessie Buckley and Rob Rinder.
Olivia Colman told the Queen she “did a great job” when she made a surprise appearance to read a letter to a packed-out crowd at a London show.
Camilla topped the bill in Letters Live at the Royal Festival Hall, an event known for its mystery line-ups of stars who read monologues from real correspondence.
Some audience members gasped and deafening applause broke out as the Queen arrived on stage for her reading.
Speaking in the artists’ bar during the interval, Colman, who read three letters during Thursday evening’s show, told Camilla: “You did a great job.”
“I love that they keep it a secret who’s coming out,” she said.
“Did you hear the shock when they heard it was you?”
The Queen admitted to Elaine Bedell, chief executive of the Southbank Centre, which houses the venue, that she had been “terrified” before her reading.
Other readers included Sherlock star Benedict Cumberbatch, Oscar winner Jessie Buckley and The Matrix actor Laurence Fishburne.
Camilla, who wore a blue Fiona Clare gown and turquoise and diamond earrings, told Colman, Cumberbatch and his wife Sophie Hunter, who introduced the readers to the stage: “All of you were so terrific.”
Cumberbatch told the Queen: “It’s a long walk to that lectern, but once you’re there it’s a lovely audience.”
Camilla read a letter from the late Queen Mother to poet Edith Sitwell in which she thanked Ms Sitwell for sending her an anthology of poems to help as she grieved the death of her husband King George VI.
In the note, dated September 15 1952, the Queen Mother, Charles’s maternal grandmother, told of how she found solace in the poems as she mourned the King, who had died that February.
Camilla, an avid reader who champions the importance of literacy in much of her work, read to the captivated, hundreds-strong audience, saying: “My dear Miss Sitwell, it was so very kind of you to send me a copy of your lovely book.
“It is giving me the greatest pleasure, and I took it out with me and I started to read it, sitting by the river and it was a day when one felt engulfed by great black clouds of unhappiness and misery.
“I found a sort of peace stealing round my heart as I read such lovely poems and heavenly words.”
The letter Camilla read concluded: “I am deeply touched by your thought of me, I love being given books, and I send you my warmest thanks.
“I am yours, very sincerely, Elizabeth R.”
Also reading letters were comedians Sara Pascoe and Sindhu Vee, TV barrister Rob Rinder and actors Daniel Kaluuya and Simon Callow, among others. The audience was also treated to performances from rock band Elbow and folk duo The Staves.
Most of the readers wore wireless headset microphones – and Camilla joked that she was “upset” that she did not get to wear one.
“They didn’t dare mic you up maybe,” Pascoe said.
The event was held in support of the National Literacy Trust, of which the Queen is patron.
Founded in London in 2013, Letters Live has since taken place in New York, Berlin and even at Glastonbury festival, with recordings of some readings going viral online.
It has brought letters written by people as varied as David Bowie and Charlotte Bronte to the stage.
