The Wolfen by Whitley Strieber

No one has ever lived to tell the horrifying truth about them. Yet even now the Wolfen are gathered in the night-dark alleys ... unseen, poised ... ready to destroy their helpless human prey. Only one man and one woman, trained cops, willing to risk their lives, stand in the way. 
Available format: epub    
This is an international best seller by Whitley Strieber and was made into a major motion picture.  More of Whitley Strieber's works can be found HERE.