There's been some interesting activity in the Bunkhouse Porch recently. Douglas Dunn has just released the 2nd volume of his Mastering The Fundamentals of the Java Programming Language ... for free! His first book, Java Rules was rated very highly and apparently this is even better.
I said the same thing yesterday about Mac OS X for Java Geeks - this is not your normal Java book. Instead, think of it as a commentary of the Java Language Specification. It basically delves into an amazing amount of detail about various features of the Java programming language by looking at and questioning how things work in the way that they do. For example, it looks at topics such as whether interfaces extend the Object class, and there's a whole section on how exceptions work underneath the covers.
This certainly isn't a book for the fainthearted, but if you’re up against some weird JVM bug or just want to delve deeper into the inner workings without reading the Java spec, then take a look. The 4Mb PDF file is available to download from http://www.javarules.com.