The first five chapters of this book provide an introduction to programming in "Java assembly language" using the assembler included on the CD-ROM. This introduction does a good job of explaining the JVM instructions, but could have used more exercises to help the reader learn and practice using more of the instructions. Additionally, the chapters, appendices, and index could have been better organized so that I didn't have to keep flipping through pages to find the syntax and semantics of instructions.
The remainder of the book explores various areas such as verification, debugging, the class file format, security, and most importantly, compiling Java and other languages into JVM code. Mostly, this material is a tutorial introduction to information provided in the Java Virtual Machine Specification, but there is some original material in the chapters devoted to compiling Scheme and Prolog.
Be aware that this book contains numerous errors; you'll want to grab the errata list from the Addison-Wesley website.