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Lots of stuff most webmasters including myself don't want to hear.
He sat hundreds of people down in front of websites and catalogued what they did and questioned them on why then published the results.
However, it is always good to get a second opinion. Reading other players (in the usability and user interface engineering community) like William Hudson, Steve Krug, Bruce "Tog" Tognazzini and Don Norman to balance some of Neilsons more die hard design and usability principles is worthwhile
Nevertheless, when it comes to usability recommendations for any kind of user interface, he's been at it for decades. He offers more than opinions - they are opinions based on solid TESTING. That makes his publications well worth the price of purchase.