At this point they might finally make an onion v3 domain. Not sure why they haven't done this yet.

You get censorship resistance and it also doesn't leave a trail that leads to your location or requires payment methods. All of which leads to deanonymization.

The main way that an adversary would identify the location of an onion site would be to shut off the power/internet in various locations. That would be an unlikely step against some book piracy, imo.