In The Java(TM) Virtual Machine Specification, Sun's designers of the Java
virtual machine provide comprehensive coverage of the Java virtual machine
class file format and instruction set. In addition, the book contains
directions for compiling the virtual machine with numerous practical
examples to clarify how it operates in practice. The book also demonstrates
the Java virtual machine's powerful verification techniques. In all, the
book provides sufficient detail to enable you to implement your own
fully-compatible Java virtual machine, or on the other hand, to just really
understand what makes the Java technology work.

Homepage:
http://java.sun.com/docs/books/vmspec/index.html
