HDX strange loss of downloaded books glitch

[stumped]

so I go to books, tap the one I was reading recently and theres a several seconds long hang up - then the book appears as a " new" book - open on page 1.

I then look at my library and see that the other 3 books that I was reading have all also disappeared from the device. I re-download them and they are each also "new" books now open at page 1. [ whispersync was off]
luckily my old bookmarks are intact in each case, but I cannot figure how / why they left the device.
the last action before this was going to pc to return a kindle lending library book, but I selected only that, did not select any remove other books from device option.
i powercycled the HDX then as a test I redownloaded a previously abandoned and removed book - that did open at the 1st page read, so the glitch as not any more widespread.
any thoughts on explanations. I cannot face trying to explain this to 1st line amazon CS who will probably just tell me to re-download the books and move on.
I do wonder if the books app used by the HDX has just updated, and the update caused the glitch as I had other issues:
when I tried to find a replacement prime lending book, my drop down menu in the app had 2 line entries for each item e.g. it sais kindle first twice, kindle singles twice...
after powercycling , much of that dropdown menu has now gone completely , I am thinking they have done away with the categories in that menu and also with the shortcut to kindle lending library ?
so f I want to browse , say science fiction, it is no longer a menu item, I have to go to main books screen, and scroll past all the pictures to reach some on screen categories... and kindle lending library is not a category.