When Breath Becomes Air by Paul Kalanathi is a work of non-fiction. Therefore most things that are referenced within it are drawn from real world experience. However, names have been changed from their real life counterparts.
At the beginning of the book though, the text "Caelica 83" is quoted before the first chapter. This is a poem written by Baron Brooke Fulke Greville, and its longer title is "Caelica 83: You that seek what life is in death."
There are also references in early chapters to TS Eliot's fictional writings, including "The Wasteland."
Most of the anecdotes and stories told throughout the book though are based on Dr Kalanathi's memories. But, for privacy reasons, details about the patients' lives, medical histories, and so on have been changed for the book.