The documentation of all classes, interfaces, methods etc. in the standard API is available online:
http://java.sun.com/j2se/1.5.0/docs/api/ (this is for Java version 1.5).
You can also download the API documentation so that you can have is locally on your harddisk, see the
download page, scroll down a bit to "J2SE 5.0 Documentation", click on the Download button.
If you want it in a book, then there are ofcourse lots of Java books available, for example
Java in a Nutshell.
[ September 11, 2006: Message edited by: Jesper Young ]