I recommend The Python Cookbook: http://oreilly.com/catalog/9780596001674. It's not a reference in the sense that it shows you the language syntax or tells you the details of the standard library, but it shows how experienced Pythonistas do things in a way that might not be obvious to you if you come from a language like C or Java, or until you're Dutch.
If you know a little Python, I recommend the Python Cookbook by Alex Martelli http://www.amazon.com/Python-Cookbook-Alex-Martelli/dp/05960... It is completely different from other types of books which are either references or tutorials which build some kind of project. It has hundreds of little recipes with discussions. The book is very readable and very useful, starting with simple functions, ending up with some pretty hairy stuff. 5 stars.