I recommend Python in a Nutshell, 2nd Edition by Alex Martelli. It is very concise, without the newbie stuff, and does point out some specifics that might surprise someone coming from Java and other languages. It's a good Python library reference to have on your bookshelf, too.

This book covers up to Python 2.5, but you could then read about the changes in Python 2.6/3.0 at:

http://www.python.org/doc/