A free book on programming python is Dive into python by Mark Pilgrim: http://www.diveintopython.org/
Another book I think is really good for beginners that want to learn Python is 'Beginning Python: From Novice to Professional, Second Edition' by Magnus Lie Hetland (http://www.apress.com/book/view/9781590599822), but that book isn't free.

This will learn you to program scripts that you can run within the terminal on your PC or on the N800 (or using launcher programs).

If you want to learn graphic user interface (what most people consider "real programs"), I think the best way is to learn PyQt (I think is it easier than PyGTK, mostly because I think the documentation is a bit better).
The book that has already been mentioned some post before: 'Rapid GUI development with Python and Qt' by Mark Summerfield is a very good one in my opinion.
http://www.qtrac.eu/pyqtbook.html
BUT you should have SOME experience in programming python before you start that book (the book has an introduction in general Python, but I recommend you look at a dedicated python book before you start the GUI book).

Other options are that you look into Web-based applications. The Tablets have a very good webbrowser and this may make it easier to create a graphic user interface (using HTML/CSS). I really enjoy creating web applications using Django (that is also Python)
A good book about Django is also freely available:
http://www.djangobook.com/
But it may be a bit difficult to do if you don't have any former experience in programing websites.

In any case, I think you should first program you application for your own PC, and then tweak it for running on the Tablet.
If you don't have any former experience it would help you gain that this way.

The big advantage of Python, PyQt is that it runs on most platforms, WIndows, Ubuntu, Maemo etc.etc.