Alan Wake's American Nightmare is a story-driven action-horror video game, developed by Remedy Entertainment and published by Microsoft Studios. The game is a downloadable follow up to its predecessor, Alan Wake, but is not a sequel. The game is an Xbox Live Arcade title and was released February 22, 2012. A Microsoft Windows version will be released May 22, 2012.
American Nightmare is set as an independent game and does not reflect on the major storyline of the franchise. It only exists to broaden and expand the universe.
Source: Wikipedia, "Alan Wake's American Nightmare", available under the CC-BY-SA License.