@Dan: I'm willing to bet that either (1) your folder is named Model, not Models, or (2) the Book class is not in the Models namespace. I would take a close look at the Book class definition, the EXACT name of the folder, namespace, class. If it's not that, then I would probably need to see your code.
@tvhong: In my mind any class that has a visual aspect to it is a control. I would say that you could use XAML to utilize all controls, but not every XAML element is a control. For example, in the App.xaml, or even the Page.xaml at the highest level a Page is not a control. Furthermore, there are things that we'll discuss later regarding animation and such that are expressed in XAML, but are not controls.
XAML is just a concise way to create instances of classes. It's not exclusive to user interfaces (for example, Windows Workflow Foundation) utilizes XAML.
@TomaszS: Yes, #70 allows the user to pay to unlock a feature. However, a trial version is implemented a bit differently but very close to the same basic procedure. The URL in the video will take you to Windows Dev Center and the instructions will be in that same area of help. Good luck!!!
@TomaszS: So, the way I look at classes is that there are two main types of classes:
There are "domain" / model / entity classes ... classes that have state (properties) and are vital to the business problem you're trying to solve. Their methods generally act on their own object instance.
Then there are utility classes that have no state, and play a supporting role. Factories, managers, orchestrators, facades. The methods act on instances of other classes (not themselves, because, again, they have no state).
Domain classes are generally not static. Factories, managers, orchestrators ... the generally ARE static