I am unsure what it means - both (1) when it is conjoined with a message saying that more are on their way and (2) when it is not. Now, at first, everything does seem clear. For one presumes that 1 means that a re-order (Amazon re-ordering from suppliers) is not guaranteed, whereas 2 means it is guaranteed (indeed, that it has happened already). What confuses me, though, is that a book can jump very quickly from normal status (i.e. 'in stock' with no other message) to being almost out of stock (i.e. to scenario 1 or 2 - actually to scenario 2). I reckon that my book (for I am an author, and I'm watching my own book) can sell out that soon. .