Apache Jackrabbit includes some kind of database and archive query mechanism called JQOM. Where is a good place to start learning what it is and how it works? Thanks in advance.
Jackrabbit is an implementation of JSR 170 and, from version 2.0 of JSR 283, JQOM is part of the JSR283 specification. A good start would be to read the spec. It is a long read, but it is definitely worth it and usually contains some clear examples.
Jackrabbit itself is fully compatible with the specification since the end of January (2010), so resources are currently insufficient, but this should improve when adoption improves.