The Java Language Specification, by James Gosling, Bill Joy, and Guy Steele. Addison-Wesley 1996.
Simply the most comprehensive book on Java out there. It is the reference text that defines what Java programs are and what they mean. Despite its weighty subject, it is a lot of fun to read.
The Java Virtual Machine Specification, by Frank Yellin and Tim Lindholm. Addison-Wesley 1998.
The bible of Java virtual machine programming.
The Java Programming Language, by Ken Arnold and James Gosling. Addison-Wesley 1997.
A tutorial introduction to the Java programming language, coauthored by James Gosling, the inventor of Java.
http://java.sun.com/ (Sun Microsystems, Inc.)
Current information on Java and related topics, including Java releases, security issues, and online documentation.
http://java.sun.com/docs/ (Sun Microsystems, Inc.)
Official Java documentation, including current information about books in the official Java series.
