— feeling confused

I was going through some book descriptions (on THAT other site) today and noticed some of the books designated as "NA".


I've become accustomed to some of the book acronyms but do not recall ever seeing this one before. Has it been around for a while and I, in my comfortable world of blissful ignorance, have never noticed it before?


So, who better to come to than a community of readers.  Can anyone enlighten me?  What does "NA" stand for?