Keith R. A. DeCandido 
Heart of the Dragon [EPUB ebook] 

A Supernatural novel that reveals a previously unseen adventure for the Winchester brothers, from the hit TV series! Twenty-three years ago, Sam and Dean Winchester lost their mother to a demonic supernatural force. Following the tragedy, their father taught the boys everything about the paranormal evil that lives in the dark corners of America… and how to kill it. Bobby Singer alerts Sam and Dean to a series of particularly brutal killings in San Francisco’s Japantown. It’s been 270 new moons since the last time, and it looks like the Heart of the Dragon is back…