And why?
To me, ElasticSearch official reference has a great in-depth explanation. However, I'd love to see more guides, examples, use-cases to be able to learn by example.
In the rare situations where something requires especially detailed debugging, the core libraries themselves are open source, and the bug tracker is equally open to help trace down any existing conversations and unexpected results.
Not sure if that's in the ballpark of what you're looking for, sorry if not.