The scariest monsters to me are always the human ones as we never expect our fellow man (or woman) to be as vicious and warped as some of them can be. If I had to pick one such "monster" from a book it would have to be Clayton Broom from the Lauren Beukes book, Broken Monsters. This guy is basically a failed artist who goes completely off the rails when his life doesn't turn out the way that he expected. When he becomes a serial killer he doesn't just murder his victims, but use his creative flair to transform them into something very disturbing. Like all good horror books, Broken Monsters does have some paranormal elements, but honestly the killer would be extremely chilling if he existed in real life.