I thought more about Wolfgang's call for more info, and looked at that
Macsyma directory.
Much of the documentation is colored by the extensions made by the
commercial macsyma. The user guide is probably a reasonable place
to start. It is here:
http://www.cs.berkeley.edu/~fateman/macsyma/docs/userg.pdf
The (other) introductory material in that directory seems mostly aimed at
getting the user to understand the graphical front end, which is different
from Maxima.
There is also a reference manual, refman16.pdf in that directory, describing
commands and the programming language. These are a superset, pretty much, of
maxima.
It is difficult to construct documentation for a complicated software
system. Just look at the proliferation of books on software topics like
Excel, Word, Photoshop,... C++, Java...
The Macsyma documentation is not as good as it should be. In fact, it might
even have contributed to its unpopularity. But there is probably information
there that is not available in Maxima documentation.