Following the launch of Alan Wake in May 2010, Remedy released two DLC packs which continued Wake's adventure in the night.
Previously, codes for the first DLC pack, The Signal, were included in new copies of Alan Wake. For The Writer, players could purchase it for 560 Microsoft Points (around $7?), but the content is now available for free to XBOX360 and XBOX One gamers who have the title.
It's definitely worth checking out if you can!