Even if you already own the second edition, you should buy the third (or get your employers to buy it for you). (Putting this first for those people who don't want to read the whole review).
I already owned the second edition - it is the most frequently borrowed book from the set that I keep on my desk. So why did I buy the new edition as soon as I could? Because it is new, and covers new stuff - like sixth normal form (which turned out to be very familiar), and the Object Class Hierarchy, which is the answer to a Corporate/Enterprise Data/Information Architect/Administrator's prayer (job title generator - for each pair, pick one - the titles may vary, but the job seems to stay the same!) After years of developing web pages, spreadsheets and documents, and trying to get people to use them, this structure finally brings it all together.
It is tempting to dip into a book like this to look things up, or to explain something to someone else (it is very handy if someone wants to know what, exactly, you mean by 'fifth normal form' - just hand them the book open at the relevent section). But if you don't read the whole thing, then you risk missing all sorts of useful stuff that gets mentioned in passing (sometimes a passing remark, sometimes getting as much as a whole paragraph). For example, there is a very useful question to elicit important information from senior management, in chapter 10. (Go and read the book to find it!)
I think that the chapter on Enterprise Data Management needs expanding. Preferably into a companion volume ('Enterprise Data Management Essentials' - any chance, gentlemen?) But that is just about my only caveat.
The second edition is still the most frequently borrowed book on my desk - but only because I don't let this one out of my sight, and I have been making people buy their own copies.
If you buy the book and disagree with me - feel free to come and tell me why at any DAMA conference!