I've been back to that library since and found out that this is not the complete story. This book also describes how this OOPS thing is a way of dealing with complexity and goes on to define what complex systems are and how to break down the complexity into manageable lumps. All very heady stuff. Suddenly, I got to realize that this OOPS thing is more than just a computer programming concept, it's a whole new way of looking at life. Components of the structure can be any arbitrarily chosen parts of the hierarchical structure.
Complexity evolves from a simple working system.
It starts as a small module, then a collection of different small modules are added. These modules are then combined in different ways to do complicated things. Then the groups of modules are combined, with other groups of modules, to do even more complicated things. Complexity is something which is built up in easy steps from simplicity. The break down of complexity is through the concept of class and object.