"Average" programmers had much more practical reason to learn assembly in the days of 16 and 32 bit x86, of DOS, Win32, and software rendered games, than today when the average programmer writes Javascript or some other dynamic language and is several levels removed from the machine code being executed, so I don't think material written about x86-64 will ever be on the level of the stuff written back then. IMO it makes more sense to stick to the high quality 32 bit texts and let students figure out the 64 bit additions on their own if they want to. x86 is conceptually simpler anyway.