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.
An engineer, a physicist and a mathematician want to catch a lion. The engineer goes to Africa, builds a cage and traps the lion in it. The physicist builds a fence around Africa and pronounces: "I've trapped the lion". The mathematician builds a cage, climbs inside and pronounces: "I am outside".