I've programmed in Java for about 8 years and I know the language quite well as a developer, but my goal is to deepen my knowledge of the internals. I've taken undergraduate courses in PL design, but they were very broad academic overviews (in Scheme, IIRC).

Can someone suggest a route to start delving into the details? Specifically, are there particular topics (say, garbage collection) that might be more approachable or be a good starting point? Is there a decent high-level book on the internals of the JVM and the design of the Java programming language? My current approach is going to be to start with the JVM spec and research as needed.