There's a completely new language reference in the process of being written: https://lean-lang.org/doc/reference/latest/ (by David Thrane Christiansen, co-author of The Little Typer, and Lean FRO member)

Some links here seem to be broken at the moment — and David's currently on vacation so they likely won't be fixed until January — but if you see for example https://lean-lang.org/basic-types/strings/ it's supposed to be https://lean-lang.org/doc/reference/latest/basic-types/strin...