I'm a fan of the first edition, but I went to the library and checked out the second edition that was released last month. The new book is longer (356 pages versus 284 pages for the first edition). That gives it room to be more complete about some things. For example, the second edition has eight pages on how to align your cantilever brakes, while the first edition has only two. The second edition also has a more complete torque table. There have to be other similar examples, but I haven't found them, yet.
He also corrects a few omissions--he adds electrical tape to the basic tool kit and a few tools to higher-level kits.
All-in-all, it's an improved book. If you are buying the Zinn book, make sure you buy the 2005 second edition.
That said, the second edition I checked out from the library lacks something my original version has--grease stains from the hours I've spent putting it to good use.