When we go back to early Java, the death of books just came from better IDEs, not even stack overflow (which wasn't there) or Google search (which had little to find). Something like IntelliJ made the very large reference section irrelevant, as you got to see source and Javadocs right there. Intellisense also found most of what you needed blindly. That was 90% of those 6 java books. You'd still see a need for specifics good explanations like, say, Java concurrency in practice for all the original low level primitives. Maybe some madman that could explain the initial implementation of the Calendar classes.

Either way, one needed far smaller books the moment all of that came in, especially because in Java, library code is very readable. Compare that to looking inside a C++ standard library: It's not impossible, but it's of a completely different nature of normal C++ we might write for a business.