[apu]

Two important components of lectures are

1. Tying facts and ideas together in a cohesive "story" so that it sticks better in people's minds. Good books are supposed to do this too, but sometimes you don't have the flexibility to choose the best books. Also, as in any story, you have high points and low; important bits and those less so. Emphasizing these differences and making sure students understand which are the central points is another main goal of good lectures.

2. Explaining difficult concepts in different (often multiple) ways so that people can understand them better. For various reasons, books have a tendency to avoid "intuitive" explanations for many concepts, perhaps out of a fear of not-quite-perfect analogies. Such analogies can be given in class with the appropriate qualifications; doing the same in a book would often require excessive legalese.

Where appropriate, these explanations often involve visual elements -- animations usually, and not just pictures (which you can of course include in books).