update: I emailed amazon re the above "vanishing book" & they asked me to talk to them - so I had a long phone discussion with a techie. NO solution found but I made them aware of "solutions" from this thread & they asked me to keep a log of all books purchased + success or fail with downloads in the hope of finding a pattern. they are also keeping in internal "ticket number" and are able to access my glitches history from my email address which is good.
the lady did confirm that having multiple installs, or remnants of old installs on your PC can cause problems, and that it can be hard to find all the leftovers.
so I would encourage anyone with software glitches to contact them - the more feedback they get the more likely they are to find something to fix