The Windy City, or Chicago, has many reasons to call itself special, but no nice big easy to point out ones. Instead, we'll discuss what I think is the reason why the author picked Chicago.
The main problem in book 5 is that the Ghost Cell is going to release some sort of chemical weapons into the air. They decided to realese it in Chicago.
Now, the nickname for Chicago and the title of this book is The Windy City. So, if they released their chemical weapon into an area with a lot of wind, the weapon is going to spread so much farther so much faster. So while being windy isn't what most people would say makes Chicago specia, in this book Chicago is special because of its winds.
To learn more about Chicago, go to the link.