[Tutor] Resources/Books for teaching OOP in Python and Tkinter

[Marie Shaw]

I am a teacher of 16-18 year olds. Since September, my classes have been learning to program in Python, using all of the basics in console mode.
I now need to teach them OOP using Python, and GUI programming using Python.
Please could someone one point me in the direction of a good book, or some good teaching resources.
I already have Python for the Absolute Beginner, by Michael Dawson, and Python in Easy Steps, by Michael McGrath. I like these books and have covered the relevant chapters in them. However, I need to learn more.
Thanks, Marie.