I have learn mostly by running an LND node myself. I don't know a lot of the technicals of how lightning works in code but I have learned how to operate a lightning node and setup channels.
What aspect of Lightning do you want to learn about?
Discussion
I would first like to learn the concepts. Basically the equivalent of understanding Bitcoin's private & public key, hierarchical wallet, UTXOs, nodes, mining, etc.
How do channels work (what happens on-chain and off-chain), what are the security tradeoffs and gotchas, what are node operator incentives, how does a transaction get routed, are some questions that come to mind.
I have never read it but Mastering Lightning is probably a great start. https://github.com/lnbook/