Spoilers ahead for Episode 4 of Castle Rock Season 2. In Castle Rock's Oct. 30 episode, young protagonist Joy Wilkes breaks into her mother's lock box only to uncover two curious items: a gun, and a CD-rom of a book called The Ravening Angel. It's unclear why her mother has been hiding this book, but it must have some significance if she's been holding onto it for all these years, and there are certainly some educated guesses to be made.
Firstly, that the book is on a CD-rom is a clue in and of itself. CDs were primarily used in the late '90s and early 2000s, so the fact that The Ravening Angel is on one might actually help place the book in time. It's possible that Annie has had this disc for 15 years, since around when Joy was born.
This is also the second time The Ravening Angel has been mentioned on Castle Rock. The pilot episode opened with brief flashes of a young Annie running to a body of water while carrying a file box that had "The Ravening Angel" written on one side in Sharpie and "Wilkes" on the other. Could that mean Annie's mother or father wrote the book? Or that she did?
Based on the page Joy reads, The Ravening Angel looks to be about a character named Prester Jack, which may be a reference to the medieval Christian king Prester John (historically, "Jack" has been used as a stand-in for "John"). Due to a lack of reliable information about his actual accomplishments or whereabouts, Prester John has become known as somewhat of a folkloric savior, and has been occasionally referenced in pop culture. In Shakespeare's Much Ado About Nothing, Benedict jokes that he would rather search for "the length of Prester John's foot" than talk to Beatrice. And more recently, Marvel loosely based comics character the Wanderer on Prester John.
This aligns with the larger Stephen King mythology. In Misery, Annie is a literature enthusiast — so much so that she kidnaps her favorite author, Paul Sheldon, and forces him to write a new installment of his romance novel series, torturing him until he completes the book. If that's what lies in Annie's future, whatever part The Ravening Angel has to play in her story, it probably isn't a pleasant one. Maybe Sheldon wasn't the first person that Annie "helped" write a book.
Based on the two instances in which we've seen its title, it's clear The Ravening Angel is not only important to Annie's past, but something she'd like to keep hidden. With Joy on the case, however, the truth is bound to come out soon.