Properties in a user created class

If I create a class and set up one property which is a variable initiated at the start of the class but not within a function or a constructor and I reference the property from the .fla in a for loop as:

Book is the class and myBook is the new object.

myBook:Book = new Book(); // (fla script).

for(var prop in myBook) {
trace(prop);
}

the .AS contains

Class Book
{

var myProp:Number = 0;

function Book() {
}

}

The trace does not return anything. However when I include myProp (the class variable) in a function (other than Book function) and call that function from the .fla then the trace works and I can see myProp.

I would be grateful if someone could explain: Do I need to include Class variables in called functions (methods of the Class) in order for them to be properties of the Class?

thanks in advance