Is there a Stack Exchange site appropriate for asking, for example which is the best Scala book for getting an overview of the language?
If so, in which site could I ask the question?
First off, thanks for making an effort to follow the rules of the sites =)
As simchona said in the comments, these types of questions would (and should) be closed as not constructive. See the relevant FAQ section that says:
There's just no objective answer to your question.
You mentioned that you've seen some of these types of posts remain open. The Stack Exchange sites are not a perfect model of their guidelines. However, generally the authoritative post on this type of question is a blog post by Jeff Atwood:
Stack Overflow (and Stack Exchange in general) has not always had such strict guidelines, and the community has a lot of say in what is closed or stays open (regardless of said guidelines).
What's to say a good book today is a good book tomorrow? Or the next day, or the next day? In 2007, The "ASP.NET tips and Tricks book" (that Jeff Atwood and others published) was a great ASP.NET book, but today, there could be better choices.
Secondly, it's somewhat subjective. There's no real 'fact' here. I may think Fooing widgets, 3rd edition is the BESTEST EVAR. You may think the 2nd edition of 'Baring Foos' is awesome-sauce. Either way, what happens when the OP picks one or the other? It's all opinion.
Finally, the answers could go on forever. There are 3.3k+ titles on Amazon for books about C# (according to an Amazon search on books for... wait for it... C#), so you could conceivably have 2,948 answers on the 'best book for C#'. Who's to say that one book is better than the others?
If you want a book recommendation, look at blogs or Amazon reviews. Stack Exchange is not the place for it.
You can ask for recommendations in chat!
Your best bet is to first see if anyone has created a chat room for what you're looking for and then if not, create a new chat room first, something like Recommend me some books. Then stay in your room, and possibly draw some people in from active rooms (like the tavern, the lounge, casual chat) with a polite, discrete post, something like:
Remember that while chat is realtime it's also archived, has RSS feeds, and can act like a forum as well. So You can create a chat room and as long as you keep it active, you can return to it days later and see what was said when you were gone. So you don't have to worry about missing any recommendations.
FYI, This is what what Jeff Atwood himself did when he wanted recommendations for a new router.
Do not ask about shopping recommendations on the main site. They will be closed. It is offtopic.