Episodes: 4.18 The Monster At the End of This Book, 5.01 Sympathy For the Devil, 5.09 The Real Ghostbusters, 5.22 Swan Song
The Supernatural Books are a series of novels, based on the actual events in Sam and Dean's life as revealed to Chuck Shurley, aka Carver Edlund, a Prophet. The books have been published by "Flying Wiccan Press: Publishers of Quality Science Fiction and Graphic Novels", although they are now out of print.
Every book title is an episode title: #1 Supernatural, #2 Wendigo, #3 Phantom Traveler, #4 Bloody Mary, #5 Skin, #6 Hookman, #7 Bugs, #8 Home, #9 Asylum, #10 Scarecrow, #11 Faith, #12 Route 666, #13 Nightmare, #14 The Benders, #15 Shadow, #16 Salvation, #17 Bloodlust, #18 Croatoan, #19 Heart, 20 Sin City, #21 Fresh Blood, #22 Mystery Spot, #23 Jus in Bello, #24 No Rest For The Wicked. This list excludes portions of season one and most of seasons two and three, notably including 3.06 Red Sky At Morning and 3.15 Time Is On My Side, both of which are specifically mentioned as having been published.
Castiel says the Supernatural books will one day be known as The Winchester Gospels. In the Christian Bible, the Gospels are the books that describe the life of Jesus Source.
Dean: Everything is in here, from the racist truck to me having sex. I'm full frontal in here, dude.
The book Dean is reading relates the events of 1.13 Route 666.
4.18 The Monster At the End of This Book
Sam and Dean discover the existence of the books while on a case - the Comic Shop guy tells them the books have a 'cult following.' The Winchesters discover an online fandom for the books, including Slash fans. They visit the publisher, and pretend to be fans of the books in order to track down the author - Chuck Shurley. Castiel reveals that the author is a Prophet who forsees events in the boys life, and that he is guarded by an Archangel (later events suggest this is Raphael.
5.01 Sympathy For the Devil
Chuck contacts a fan of the books, Becky, and reveals that they are not fantasy but fact. He gets her to deliver a message to Sam and Dean, not knowing that the message is a false prophecy set up as a trap by Zachariah.
5.09 The Real Ghostbusters
Becky invites the boys to a Supernatural Convention organized around the fandom for the books. After the Convention, Becky remembers that in the book 'Time Is On My Side', it was revealed that Bela gave the Colt not to Lilith, but to the demon Crowley, giving the boys hope that they might retrieve it and use it against Lucifer. Chuck says he has a backer who will help him publish moer books, but Sam sugests in no uncertain terms that this would nto be a good idea "we have guns - we'll hunt you down!".
5.22 Swan Song
Chuck writes the final Supernatural book.
The Supernatural books act as a metafictive device in the text - that is they allow the writers to make commentary on the actual Show, its writing and the fandom. It is an example of is an example of breaking the fourth wall.