After you get some Java under your belt try Effective Java and Java Concurrency in Practice. Also I read the Javadocs for all the built in libraries like java.util [1] and the Javadocs for the stream api [2]. After that I read the language spec, and the memory model spec.

1: https://docs.oracle.com/javase/8/docs/api/java/util/package-... 2: https://docs.oracle.com/javase/8/docs/api/java/util/stream/p...