To me, this is an excellent addition to the main book.
The book is broken down into four main segments. The first three are all about the monsters, as it should be, with the last segment about adapting monsters to player characters.
The first segment covers Ghosts and Spirits. It goes through several that you see in the TV series, such as the Hookman, Bloody Mary, ect, as well as a couple you don't see or are only mentioned in passing, such as the Rawhead or the Angiak.
The second segment covers Demon's and Angel's. This goes through and gives several examples of the heavy hitters you see in the TV series, such as Llith, Azazel, Alastair, Castiel, Uriel, and Zachariah. It gives more precedence to the Angels than the demons, as Angels are completely new to this book, and Demon's were covered in the main book already.
The Third Segment covers your various monsters, such as the Werewolf, Skinwalker, Rugaru, and others. This segment does have a few monsters that are not mentioned anywhere in the show (Manitou and Mothman) and while the Manitou seems to follow Supernatural lore, Mothman is....a step outside of the box. Not one I will be using personally, but that is just personal taste.
All three of the first segments individual entries the lore on them, as well as a sample monster and it's story, ready to be fleshed out into a full scale hunt. The only chapter that is light on this info is the Angel's and Demon's, as most of the ones that are mentioned there are figures that played highly in the TV series, so the focus in that chapter is more on guest appearances of those baddies than full fledge hunts for them, though you could use the info to write a different ending if you chose.
All three of the first several segments also give you write ups on ideas how to run a campaign centered on those types of creatures, which is nice, but to get the proper feel for the show, I feel you need to include them all, to some extent or another, but that is just an opinion.
All in all, good book and good pdf. I haven't had the problems printing or viewing that was mentioned in the other review, though i will admit it does take a couple seconds to turn pages sometimes, but not nearly as long as was mentioned. Printed fine, as well, so no problems there for me. All an all, a good book for a supernatural fan and for the RPG