If you created the Book class directly in the Models folder, it'll probably be in a different namespace.
Where you get the squiggly red line under the Book type just right-click the word Book and pick Resolve -> using
some namespace.
Or you can just the "using" line at the top of your code yourself:
using YourProject.Models;
Not, it's not a System namespace. It's whatever namespace is specified at the top of your Book class file.