A GTK+ 2.0 (!) example for a drawing program with backing pixmap and extended input for tablets: http://developer.gnome.org/gtk-tutorial/stable/c2422.html
Very good Python tutorial for GTK+ 3.0: http://python-gtk-3-tutorial.readthedocs.org/
The GTK+ 3.0 API documentation: http://developer.gnome.org/gtk3/stable/
An explanation for of how to the C GTK+ 3.0 API documentation to Python https://live.gnome.org/PyGObject/IntrospectionPorting#How_does_PyGI_work.3F
The PyCairo reference: http://cairographics.org/documentation/pycairo/3/reference/index.html
The Poppler API reference: http://people.freedesktop.org/~ajohnson/docs/poppler-glib/index.html
Twisted (networking) documentation and reference: http://twistedmatrix.com/trac/wiki/Documentation
Twisted Perspective Broker (aka RPC) How-To: http://twistedmatrix.com/documents/12.0.0/core/howto/pb-usage.html