I don’t know what the difference is between downloading and sharing libraries, but check out Soulseek. It’s a little file-sharing program that lets others download directly from your music folder and vice versa. When I couldn’t find an album on any torrent tracker, some hero on Soulseek had it. The main drawback compared to torrents is that you’re limited to the upload speed of this one person, you can’t connect to a dozen people with the same files and combine their power.
I really felt like all the good stuff was only on private trackers the last few years. A few of the biggest public trackers also went down, that certainly didn’t help, but even before that I had a hard time finding some older, rarer things. If piracy gets bigger, I hope to see more reliable public trackers again with bigger catalogues.