A monster manual for Call of Cthulhu.
But it is more than that. CoC is not a monster hunting game, it is often a fact or truth finding one and so the monsters here are placed in context in the world. There are reasons why they are here and they have agendas. There are myths about them all so there is plenty to do with the information in this book and the players my never even encounter or fight one of the monsters (but that would be a shame). The art is a mixed lot, but I think that is a plus in this sense, since there is a "pieced together" feel the publisher was looking for. A monster is mentioned here in one context then on the other side of the world decades later in another. The book does give that feel. There are some photos as well for and added feel of this being the work of investigators of time gone by.