https://learning.chaincode.com/ all day! It's freeeeee. If you can't wait for or commit to a class, the course materials are here: https://chaincode.gitbook.io/seminars/. It's more protocol level than API level, but the APIs are relatively trivial. Fantastic program.

Mastering {Bitcoin,Lightning} are good but dense. They can feel a lot like encyclopedias.

If you're anything like me, just build something. This is exactly how SN started. "I want to build something on Bitcoin. Oh, Lightning seems like an unlock, what can be done? Oh, Sphinx Chat and @paul are really onto something. Man, I've been wanting a Hacker News for Bitcoin for years now; it'd basically be like a bit devs online. Here we go..."