Python in a Nutshell is a excellent reference book, all python programmer must have one in the desk.
The online documentation for the python language is good, but some times is difficult to search for a feature of the language, in this book is easy to find anything about python.
This book complements perfectly the official library and language references, and covers almost everything you will ever need.
I Learn python with this book, but i only recommend the book for people with knowledge of other programming language, if you are new in programming and wants to learn python this book is not good for you (try Learning python by David Ascher and Mark Lutz), if you already have programming skills this is the perfect book for learn python.
The author explain the language in a very simple way with good examples, and don't wast time explaining the basics of programming, the book starts with a introduction to the python language, in chapter 2 the author explain the installation of various python interpreters, from chapter 4 to 9 the author explain the core of the python language, in the rest of the book the author cover the python library and extensions modules, network and web programming with python and in the last section the author talk about the various ways of embedding and extending the python language, in my opinion the book is well written and very easy to read.
I highly recommend Python in a Nutshell, for all programmers who wants to start coding in python, and for all python programmers as a language reference.