Like most programming tasks, the best place to start is to just dive in.
The most obvious place to look is the xmlwrapp header files. They are clean and well documented. When any document does not agree with the header file, the header file will always win.
The header files are documented using the Doxygen syntax. The comments are then used to build the API reference manual.
If you need specific help using xmlwrapp, you can try the xmlwrapp-users mailing list.