Question: **I imagine that no, they aren't, because every process has its own memory space, of course.**

But how does the whole JVM thing actually work? Is there a separate JVM in a separate process for every Java program that I launch? Do Java programs running in a system share anything at all? Are there differences between OSs and JVM implementations? Can I *make* programs share variables (i. e. directly through the JVM rather than the usual IPC mechanisms)? Are there more exotic one-process JVMs for special purposes?

Generally, what are recommendable reads about the guts of JVMs? The [spec](http://java.sun.com/docs/books/jls/third_edition/html/j3TOC.html)? The source code of some implementation? Websites? Books?

Answer: 
No, static variables aren't between JVMs. Yes, there's a separate JVM process for each Java app you run.

In some implementations I believe they may share *some* resources (e.g. memory for JITted code of JRE classes) but I'm not sure. I believe there's ongoing work to enable more sharing, but still in a robust way. (You don't really want one JVM crashing to affect others.)

No, you can't make programs share variables transparently.

I believe there are books about the JVM, but I can't recommend any. You'd probably be best off looking for HotSpot white papers for details of that. [This one](http://java.sun.com/products/hotspot/whitepaper.html) is a pretty good starting point.
Depends on the implementation, but yes there are ways things are shared. There is/was work underway to change that, and Sun (via Apple) has done a lot of work in sharing data between instances of the VM. There is a link that discusses some of that [here](http://www.javalobby.org/java/forums/t72620.html?start=30).

To do any sort of sharing requires the VM implementor to do it, you as a programmer cannot do anything to make it happen.

The VM spec is [here](http://java.sun.com/docs/books/jvms/second_edition/html/VMSpecTOC.doc.html), there are some (older) books about it as well. You can also look at the source for [Kaffe](http://www.kaffe.org/) which is pretty small.