Any eBook preview handler that works in File Explorer should also work in Opus. There's also a native viewer plugin API if anyone wants to write an Opus-specific one.
eBooks are a thing now, here to stay. I'm not up to writing a preview handler and would certainly pay extra bucks for this feature, say for version 13
I did look at the sumatra page and apparently it's open source, does all these formats - Perhaps you or some other capable programmer could have a look ?
.epubs are just zip files - add the extension to Opus prefs and you can double click them to view the contents, then preview the .html files. Not the same but good enough for a quick preview surely.