So, is it fair to say Apple's ePub support (or should I restruct that to iBooks?) is a bit like Microsoft's first few stabs at supporting HTML? -- it was an open standard, so long as you did things Microsoft's way and wrote for Microsoft's non-compliant browser and accommodated Microsoft's proprietary modifications and extensions.
Ta for the link to Liz Castro's blog -- I hadn't encountered her before. It's a bit perilous for wide-eyed ones like me, who can't really tell which parts of what I'm reading are Apple-specific and which are more generally viable, but I guess that will become clearer with time. So, thanks.