Lecture #2 introduced the behaviour-based approach to AI.
This material isn't really covered in the textbook (the lecture material was drawn from Chapter 2 of Nilsson's "Artificial Intelligence: A new synthesis".
The classic article on the subsumption architecture is
Brooks, R. A., "Intelligence Without Representation", Artificial Intelligence Journal (47), 1991, pp. 139-159For a full understanding of this material you should read this paper. For those more interested, a recent paper is:
Brooks, R.A., " From Earwigs to Humans", Robotics and Autonomous Systems, Vol. 20, Nos. 2-4, June 1997, pp. 291-304.