Just to add ....
I just tried adding a description in Calibre (the field in the meta data is called Comments). It doesn't show up at all in the Kindle, or the Kindle for PC app. But I can see it clearly in the Mobi Pocket reader.
I don't suppose that helps you much. In any case, I can only do that with a non-DRM title. Just thought I'd mention it in case anyone is interested.