Oh my goodness wonderful request...Loved this...one question what is this letter from? You said it's from a book? And can I get my hands on it? Okay so it was three questions...
Hello! Thank you! Yes, it was from a book called Doctor Who: The Companion’s Companion. Clara wrote it full of quizzes and other information on the Doctor’s past companions. She then hid it all in the TARDIS. He found the book and added in a page addressed to her, and then put it back in her hiding place.
Here’s photos of the book cover, the page Peter read out loud, and then two YouTube videos of him on stage. The P.S. got cut in the first one, so I added it in from my original video.