Thank you Manichean. Sorry for putting it in wrong place.
The books all worked OK before I upgraded to ver 0.8
I am not comfortabe with trying to do anything with HTML.
There were .prc format - no conversion needed
I will look in Mobi forum but for now would prefer to revert to previous version of Calibre - if I knew how to do that - can you tell me where to find instruction on this?