Whats the best way to find out about the Apache API and data structures?
To get the best understanding, there is no substitute from reading the code and the examples of modules and filters which come with the Apache distribution. An extremely useful tool is Doxygen. Just download and install it then run make dox in the top level Apache distribution directory. It will generate very useful hyperlinked documentation about the APR and Apache structures. The output of running Doxygen on the APR is available here. The output of running Doxygen on the Apache source is available here.