How do people safely download from these torrent sites? Isn't there a risk that you'll download something you wouldn't want on your computer? Yet I hear of many people actively using them, so there must be more to it.
It's easy for others to see what you're downloading: https://iknowwhatyoudownload.com. So if you're unsure if the torrent is legitimate, I'd probably avoid it.
When you download the torrent file, you're trusting that the provider of the torrent file, anna's archive in this case, is giving you legitimate files. Although I'm not sure how you would disguise undesirable files like viruses as ebooks; the file format would be a giveaway.
Given the context, they're probably referring to epubs. Files could contain just generic malicious javascript or malware that attempts to exploit vulnerabilities in specific ereaders.