If your Supernatural fan is a bit more of the solitude type, don't forget that there's a LOT of books out there. From behind-the-scenes & episode guides for 1-7 to "additional adventures " taking place between episodes. My personal pick is anything by Keith R. A. DeCandido or "The Roads Not Taken" a choose-your-own-adventure style book that's actually pretty awesome in its execution. If you need the ultimate coffee table book (emphasis on ultimate) there's the Essential Supernatural. I have a copy I still haven't finished (mostly because it has so much awesome in it, looking inside is like staring at the sun). [Emphasis added]
For the record, my three novels in the series are Nevermore (the boys deal with Edgar Allan Poe-related murders in the Bronx), Bone Key (a demon supercharges the spirits in Key West), and Heart of the Dragon (a monster recurs every twenty years in San Francisco, and it's fought by the Campbell family in 1969, John Winchester in 1989, and Sam, Dean, & Castiel in 2009).