Harmony developers are responsible for designing and building a large complex system. Here are a number of books that are kept at people's right hand, covering the skills and reference material relevant to building a production quality runtime environment.
Feel free to add your favorite!
Gordon, Rob. Essential JNI: Java Native Interface. Prentice Hall. 1998.
A good text to explain the principles of programming to the JNI, with a handy reference section at the back.
Oaks, Scott. Java Security. O'Reilly Media, Inc. second edition 2001.
This book provides present a overview of the Java security architecture and security classes, in addition to a wealth of detailed information and code examples for specific implementations.
Bloch, Josh. Effective Java. Prentice Hall. second edition 2008.
Goetz, B. et al. Java Concurrency in Practice. Addison-Wesley Professional. 2006.