If we ended up with something like The Rust Book but for Scala, and available free online, that would be amazing.
We do have several excellent Scala intro books, that cost money, have remained in print and been updated for new Scala versions for some years now; further updates are already in the works. The most frequently recommended are “Programming in Scala” (Odersky/Venners/Spoon, from Artima) and “Programming Scala” (Wampler/Payne, from O’Reilly).
In the free-online-book space, check out Underscore’s series at https://underscore.io/books/ . It includes two language intro books, Essential Scala (for people who can code already) and Creative Scala (for new coders).
For reference documentation, this link has already been mentioned, but I want to highlight it again:
There is a great deal there already; there is also great room for improvement. Updates to existing sections are welcome via https://github.com/scala/docs.scala-lang/pulls .
Also welcome are entire new sections. There are multiple guides on that page only exist because some individual saw a need, wrote them, and submitted them.
I don’t want to dismiss any talk about longer term solutions (e.g. about how documentation work could be better funded) and grander schemes (about writing entire new books). I just want to also emphasize that a structure is already in place where interested individuals can make useful improvements now, today.