One thing I liked about the book was it emphasised the conceptual difference between 'interface' and 'implementation'. "Interface" is "what you need to know to

One thing I liked about the book was it emphasised the conceptual difference between 'interface' and 'implementation'. "Interface" is "what you need to know to use the module". Some module is "complex" if you need to know more than the interface suggests in order to use that module (e.g. you have to know implementation details); or if the interface requires irrelevant details.