From the book's foreward:
Players in horror campaigns are a little too accustomed to the nightmares their characters face; even the most eldritch of tentacular horrors is less intimidating when you know exactly what it is, because your PC has faced it before.
New times demand new nightmares. This, therefore, is a book of horrors, not a manual of monsters. The horrors are nightmarishly intimate, often created from human vice, or let loose by human greed. They show us the ugliness that underlies reality. They are the crawling things under the rock of the everyday, sane world. Consequently, we've detailed our creatures in depth.
Each one has its own agenda, its own reason for existence and its own legend. We've made these creatures unusual, frightening and bizarre, yet sufficiently comprehensible that they players realize they are up against something intelligent, if inhuman.