As with The Gathering, if you read this trilogy, you'll want to have the third book (The Rising) ready to go, because The Calling ends, not really with a major cliffhanger (although there is very little resolution), and the story is far from over.
We spend the majority of this book learning more about the characters while they are on the run, but in my opinion, not quite enough about some of them. The book is about Maya and her friends, but it focuses mostly on Maya (and her friend Daniel), and I was hoping that it would flesh out just a little more about why the other kids (Nicole, Hayley, Corey) were so special. Here's hoping we find out a lot more in The Rising.