If you're willing to buy a book in hard-copy now, do get
Core Python Programming, Second Edition - Wesley J. Chun. Its written for Python versions up to 2.5. (2.6 is not much different, but 3.0 is VERY different and non-backwards compatible)
For a free e-book instead, get
Swaroop C.H.'s A Byte of Python.
As for the GUI part, I suggest using the Nokia's
Qt Software (PyQt4 by
Riverbank Computing, for the Python version). Its really easy to get going with, having its base set in Object Oriented development methods.
Tkinter is old, I'm not sure if you would enjoy it, though its pretty simple.