JPOX main goal is to implement Sun JDO 1.0 specification, so to start learning JDO here are some cool links
http://jdo-tools.sourceforge.net/
http://www.ogilviepartners.com (You can download for free a PDF version of the JDO Book from Robin Ross)
Good places to ask questions about JDO are the following sites. I suggest you to register in both. The JDO experts are there!!!
http://www.jdocentral.com/forums/index.php (Forum)
http://groups.yahoo.com/group/javadataobjects (Group)
Tools !!! There is an open source tool for Eclipse IDE that can help you while developing an application using JDO.
http://www.tutorial-omondo.com/jdo/judo/first_flash.html (demo)
http://sourceforge.net/projects/eclipsejdo (EclipseJDO site)
Also, dont hesitate to use this list to ask the other developers. (jpox-developers@lists.sourceforge.net)