I'm fairly new to epubs and calibre. Unlike detecting broken epubs (which causes some devices to refuse to view epubs) this is more an overview of editing or authoring your own, usually converting text files or the like.
That was a lot of work. You better bloody appreciate it! The photo on the cover is actually worse than that. I cleaned it up a bit, but you can't make a silk purse out of a sows ear. I even had a crack at getting an AI model to generate a similar, higher quality substitute, but failed.