Our textbook focuses on program development rather than the many details of the Java language itself. It rightly assumes that there are many books and online sources of information about Java. So this course page contains useful links to such Java resources, to fill the gap. When I want to know just about anything, I simply use google.com to search. It works fine for virtually any kind of question you might have about the Java language.
At last count there were over sixteen hundred books (!) on the Java language, so picking out a few to suggest here is not easy!
Here's a complete on-line edition of Bruce Eckel's Thinking In Java 2nd Edition (cached on the CCS site). (Added June 29, 2001). To see the source code, as a file, if you see a reference to it such as "c02:HelloDate.java", you'll need to add the following directory and file after the tij-2nd/ path, to give, e.g., http://www.ccs.neu.edu/home/futrelle/teaching/java-resources/tij-2nd/c02/HelloDate.java (I was unable to find an index page for the source code.)
Javadoc is one of the Java platform utilities that will generate a collection of pages for you from your comments in your source code. The comments must be of a certain form and appear in right places for this to work. At some point in the course you will be required to generate these, so start experimenting with them now (not just reading about them). Here is the course page on Javadoc.