The GOG release of the game typically includes the manual and "The Alan Wake Files" as part of the bonus extras.
The book serves as a deep-dive into the mythology of the first game, offering perspectives that Alan himself didn't have access to. Key sections include: Have you read the Alan Wake files? : r/AlanWake
If you are looking for a legitimate way to access the digital version, there are several official paths:
This "investigatory compendium" isn't just a manual; it’s a canon document written by a fictional character named , who experiences haunting visions of Alan Wake and travels to Washington to investigate the author’s disappearance. Where to Find the Official PDF