This is a good addition to the story of Supernatural. It fills in some of the history of John Winchester and the boys providing some interesting titbits of information that, so far, have not been in the show or the graphic novels. I say so far because there is a new graphic novel due out at the end of this month.
If you want to know more about Sam and the effects the Yellow Eyed Demon have had on him, check this out. Want more info on Dean and the car, look here. Want to know why John was so Paranoid and Dean so protective then buy the book. Just want to know more - there is more in this book!
Supernatural lover or obsessive check it out because there is a lot of back story that fills in some of the gaps left by the show.