Well, I think that the book contain new content related to Windows 7 and Windows Server 2008 R2. Other parts are probably very similar (or the same) as in the previous editions. I think that the "problem" is that every new version of Windows is not written from scratch. So, it is enough to keep the book up to date with the newest verzsion of the OS and to fix typos and other mistakes. Paperback of the 6th edition lies next to my bed, however, I seem to be unable to find time to look into it now.
Everything is about money
I agree. But in this particular case, I think that Mark, Alex and David did great things in the past (Sysinternals utilities for example), hence I see no problem with paying them indirectly some money by buying their book which I found to be very helpful in some cases.
I have no experience in writting and "selling" English books where the market is, maybe, large enough to generate reasonable amount of money. I have experience with writting and "selling" Czech book that focuses on on Windows kernel. Because Czech Republic really isn't a large country with many people living there, the market seems to be very limited, so I expect no money that would reward my work on the book. And my expectaions seem to be true.
However, I did not want to write about the markets. I wanted to say that writting a book really is not an easy job. It is very time-consuming with uncertain result. and they are not really happy (which is easily understandable because many things are about money) when the book is getting late.