Thanks for the links, Rubén. Some reading there for me.
I hear what you say about the preserveAspectRatio="none" setting only working when the image is the same pixel size (or ratio) as the screen, which it may not be on other devices. That makes sense, of course. What confuses me, though, is that on my two Kobos, with the preserveAspectRatio="none" setting, the cover is the same size as the full screen, but if I use preserveAspectRatio="xMidYMid meet" (which, if I understand correctly, you're saying will work universally), then on my two Kobos the cover image *does not* fill the screen, even though it's the same pixel size. With the "meet" setting, the image is compressed laterally and there are thing margins up each side of the image where the background shows through.
So it seems to me that the "meet" setting isn't working as you described. The rest of the code is identical to what you posted here. Is there a reason this doesn't work for me?