I have just google'd this: "the Kindle Edition of this title is not currently available for purchase" (surrounded by quotes); turns out this is the message that gets displayed when there is a newer edition of the Kindle ebook available (i.e, it out to basically mean: "we no longer sell this one because there is a newer edition available; go buy that." )
The title I'm talking about, namely
https://www.amazon.com/Discrete-Time...Q?tag=mr060-20 , though, is already the most recent. I wonder why they think it isn't.