I recommend starting research with link directories. There, too, of course, there will be garbage and outdated links, but there is not much choice. Of the English—speaking, the most famous resource is The Hidden Wiki, in Russian — “Godnotaba”. In addition, there are more than a dozen collections of links — see, for example, OnionDir and Oneirun.
Beware of fakes! Popular collections of links are often forged, replacing the addresses of resources. There are fakes at the “Godnotaba”, so be careful.
Rampant piracy and cheap book readers have made buying books unnecessary for many. But the copyright holders are unlikely to put up with this. Therefore, there are fewer and fewer links to download books in the clearnet. In the darknet— it’s another matter: there are “Flibusta” and “Verbal Hero” to choose from. The choice there is so huge that it seems as if there is everything at all.
Back in the early-mid-noughties, no one thought of banning torrents. People downloaded books, movies, games, TV series and even textbooks and sometimes did not even think that they were breaking the law. This situation is gradually changing, but the darknet is becoming a new haven for piracy.
Torrent trackers and search engines like RuTor and The Pirate Bay necessarily have links in onion, which give users the opportunity to ignore prohibitions and restrictions.