JJ Abrams supposedly got inspired after finding a book that was left purposefully for someone else to find. He channeled that inspiration into a book titled "S." by Doug Dorst. I'm not a huge fiction reader but I have found the book really fascinating. It's presented as a fictional book called The Ship Of Thesus with notes in the margins from two people as they read it and passed it back and forth.
Toby Litt wrote a novel called Finding Myself where the book is presented as a diary account of a holiday with some friends of the fictional protagonist, but it includes notes from the editor of the character who has written the diary.
