Canon or Not?
As for whether the Supernatural tie-in materials are meant to be canon...it's really all up to the reader. The comics and novels open up a huge universe that we don't necessarily get to see on the show... Kripke has apparently said that the Supernatural materials are not canon, they're fun fodder. But their stories can be taken as canon if the fans want.
Canon refers to all the information arising from the original, or officially sanctioned, material.
In Supernatural, in addition to the TV Show canon covers material in the
- Official Site
- Supernatural novels: Nevermore and Witch's Canyon
- The Official Companions: Season 1 and Season 2
- The Supernatural Book of Monsters, Spirits, Demons and Ghouls
The notion of what comprises the canon is subjective, and both fans and the creators of a body of material will have differing opinions on the topic. On this site, canon from sources other than the TV Show contains a superscript tag to indicate its source, so readers can ascribe their own hierarchy or value to the information.