There are new chapters on the Tcl Web Server, building Tcl/Tk and extensions from the source, and internationalization. Several chapters got major overhauls, including the ones on regular expressions, eval, C programming, and the Web browser plug-in. The rest of the book got updated to 8.2, and I worked to improve many of the examples.
However, maybe even more important is that the book uses a new binding so it won't fall apart (really!). It may sound silly to talk about the cover instead of what's inside, but I've received quite a few complaints about the binding in the 2nd edition. I finally found the worst copy I could find and sent it to my publisher. That got their attention and they've switched to a lay-flat binding ("RepKover" binding).
Another issue with a large book like this is finding what you want. To that end I overhauled the index. It is now three times larger and I hope it will help you find the information you are looking for.
Main book page