Hi Tim,
I have both the 2nd and 3rd editions (pink cover is 3rd, 2nd green) and I think that the 3rd edition is actually not as good as the 2nd.  It feels like they have opened out the functionality that they cover, but that the extra content happens at the cost of poorer coverage...

Also, it doesn't cover that much of the new media stuff, ie. no mention of Wiki or Muse.  If you can get hold of a 2nd edition in good nick then I'd recommend it, otherwise a combination of the Emacs Wiki and the info pages should help you in most situations...

Just my opinion of course...

Richard