[electrum]

There are two books we recommend to every developer:

Effective Java: http://java.sun.com/docs/books/effective/

Java Concurrency in Practice: http://www.javapuzzlers.com/

Everything else is covered by the excellent online documentation, including the language and VM specification books: http://docs.oracle.com/javase/specs/

Java Puzzlers (http://www.javapuzzlers.com/) is a fun read that highlights many corner cases of the language and might be helpful for code reviews and debugging.

[edwinnathaniel]

Better update the title with this one:

Java Performance: http://www.amazon.com/Java-Performance-Charlie-Hunt/dp/01371...

Java Puzzlers is helpful if you're doing low-level programming using Java. I find it to be less useful for app development.