Popular web frameworks for Scala that I know of are Play Framework, Spray, and Scalatra. Play Framework is my go-to framework because it very powerful and has a lot of functionality that I need and want.
A good resource for learning functional programming concepts in scala is a book called Functional Programming in Scala. The book has taught me a lot and is great at explaining concepts with small exercises along the way.
One great thing about Scala is vast amount of libraries you can tap into. You can use any library that's available in Java in addition to those that are strictly for Scala. Some great libraries for Scala are Akka (concurrency through actor pattern), Slick (SQL library), and Spark (data processing).
Can I ask what language(s) you already know, if any?
Go looks more promising as it's good for writing infrastructures,can attract some ppl/projects away from C/C++/python. But when it comes to complex business softwares, I'd say Java is still the way to go.