If this were a TV show it would begin with "for Mature Audiences Only".
That being said I enjoyed this book,parts of it take place on this earth and other parts jump to an alternate earth with same characters.That can be a little confusing at first but once you get to know the characters it's easier to distinguish between the two.
Now what the book is about is aliens that take over the human body kind of like Invasion of The Body Snatchers except that after a while they break out of the human skin.The book refers to this as The Darkness,or something evil.
This is always pretty graphic and the monsters that come out are not pretty.
But the character development alone is what keeps me in this series.