Randell’s got another (large) book on ARM assembly. He’s also got a handful of other great books on software engineering in general - he’s more than aware that “there are other 64-bit OSes, CPUs and assemblers for them, and people do use them.”
This particular title, “the art of assembly” has been around for a while. This x64 version is the latest iteration - it’s a great book compared to the previous version which introduced and focused on “high-level-assembly”, basically a collection of masm macros that effectively constituted a small language.
It’s fair to say that anyone even remotely interested on x86/64 assembly is aware of AoA.