I typically only use the spec (https://www.bittorrent.org/beps/bep_0003.html) because it's simple enough, but in case of doubt I guess a third party resource is good. I found https://blog.jse.li/posts/torrent/ to be quite informative about it.

I used to work on a fork of https://github.com/jackpal/Taipei-Torrent with custom features, I found its code to be easy enough to understand