I'd have said "Python in a Nutshell", absolutely. The only problem is that I'd have said that several years ago - the most recent (second) edition was released in 2006. Actually, I'm surprised it's quite so long ago - I thought there was a third edition, but Google seems to say otherwise.
The official manual is good too - if you download a PDF rather than relying on the stuff bundled with the interpreter. See for example http://docs.python.org/download.html
The docs bundled with the interpreter seem unnecessarily difficult to use (problems finding the content due to linking issues). The PDF is I think the same document, but being a single PDF, there's easy workarounds for the navigation issues.