In each of the books, our heroes have had to face the agents of their greater enemy. These agents are simply referred to as Them. We are learning that to each character They have a different appearance. We are also learning more about Their abilities and limitations.
One thing that I try to stress in each of the novels is that They are not the ultimate enemy that our primary characters will eventually have to face. Still, They are in the here-and-now and must be fought continually for our heroes to survive. Their job is to prepare the way for the greater evil to enter our world.