These nice old books have apparently been missing since the entire web crawling with references to the original location. There is also an Swedish version here.
Look, a class
Look polymorphism, where the classes representing eatables all inherit the abstract base-class Food. This means that they can all be used by the client Sam, who only calls the method Eat() in the interface Food. So Sam doesnt have to know about the different kinds of Food, and is able to handle them all without extra program code.