I think that, back when Supernatural first began, there was a chance that it could have veered off into “urban legend of the week” pablum, with episodes that were basically riffs on “I Know What You Did Last Summer” and the like. I think that, if the WB had had its way, the show definitely could have gone in that direction. Indeed, I wonder if that’s what the WB anticipated getting when they greenlit the pilot. Instead, what we have been gifted is something much more mature, intelligent, and compelling.

You may be interested in The “Supernatural” Book of Monsters, Spirits, Demons, and Ghouls, by Alex Irvine and Eric Kripke. It’s available on Amazon and elsewhere. I only just learned of it a few minutes ago, but it looks like an interesting read. It’s apparently written in the voice of Sam and Dean as collection of their notes, observations, and memories, interwoven with sections of John Winchester’s journal. It describes the various creatures encountered over the course of the show (or at least, over the course of the first season and half of season two, which is when it was published) and gives more background info on them.