The JDK from java.sun.com. Even if you choose to use a third-party development environment, its always a good idea to have the JDK on hand in case you come up against what might be a compiler error. The JDK is the touchstone, and if there is a bug in it, chances are it will be well known.
The JDK documentation from java.sun.com, in HTML. I have never found a reference book on the standard Java libraries that wasnt out of date or missing information. Although the HTML documentation from Sun is shot-through with small bugs and is sometimes unusably terse, all the classes and methods are at least there. People are sometimes uncomfortable at first using an online resource rather than a printed book, but its worth your while to get over this and open the HTML docs first so you can at least get the big picture. If you cant figure it out at that point, then reach for the printed books.