the "old" Lean reference is very good. i don't understand why it was replaced by an incredibly dry "new" reference (which i personally find mostly unsuitable for learning the language). but i'm glad this is still up: [sebasti.a.nullri.ch/lean4/doc/](https://sebasti.a.nullri.ch/lean4/doc/ )
[What is Lean - Lean Manual](https://sebasti.a.nullri.ch/lean4/doc/ )