Surprisingly, the answer here is not the usual, emphatic cry of O'Reilly!. O'Reilly's Programming Python, is a horrible, bloated, rambling attempt to describe the language that's anything but concise (1256 pages!?). The author (Mark Lutz) has this weird obsession with object orientation under Python, which drives me insane (Python has even less maturity and coherence than C++ in this area, if that's possible), and is far better when used as a procedural language. Surprisingly, Lutz's other Python book, thePython Pocket Reference is a gem, and deserves its place on the desk (or in the back pocket of), any serious Python programmer. Since it's intended as a pocket reference, though, it leaves out a lot of important third party libraries and useful techniques.

Enter New Riders, with their seminal text, the Python Essential Reference. It blends the conciseness of the Python Pocket Reference with useful examples, well thought out discussions of technique, and documentation of important topics like threading and sockets, to name a few. This is THE book to get to learn the language, and use as a reference. It also has the enthusiastic endorsement of Robey, who tossed it towards me after I asked one too many stupid questions about sockets...