The very basics: Official Docs - to me this is the most favorite docs I have seen in my life among all the programming languages. No kidding, they are written absolutely great. Also, there's a quick introduction for Java devs.
Videos: check out RockTheJvm channel on Youtube. He also has a website with lots of courses varying from language core features to all the popular frameworks and tools.
Spark specifically: after you're familiar with core concepts and ready to look into Spark there's a very good course on Coursera explaining it. I want to warn you that even though RDD is a fundamental structure and looks a lot like Scala classic collection, nowadays Dataframe is dominating the industry. So, I would spend more time reading and learning about Dataframes rather than pure RDDs.
These are what I've personally read and watched. I bet other people in comments will give you a lot more resources on top of that