I want to learn more about Bit Torrent and want to build a client, to learn more about protocol and networking. Any resources to get started (other than Wikipedia page and official spec)?
You can start by going through the docs of popular open source bit-torrent clients like qBittorrent (https://github.com/qbittorrent/qBittorrent) or Transmission (https://github.com/transmission/transmission).
Join the respective IRCs on freenode, observe the chats and ask specific questions when you don't understand something and Google can't give you good answers.