Lecture #15 discussed communication and communication languages.
Notes are available in two formats (note that the numbering on the slides diverges from my numbering of the lectures).
This lecture covered Chapter 8 of the textbook. You should read through Chapter 8 in detail.
The reading to go with the class is:
Yannis Labrou and Tim Finin, "Semantics for an Agent Communication Language", in Intelligent Agents IV: Agent Theories, Architectures, and Languages, M. P. Singh and A. Rao and M. J. Wooldridge, eds., Springer-Verlag, 1998.which you can find on Citeseer.