I wish I had run across these tutorial links a couple of years ago when I was first getting started with jME – they provide invaluable explanations of 3D math concepts that would have kept me from certain confusions along the way in applying various jME methods. I think that they will be very helpful to others here who want to firm up their 3D math and conceptual understandings of thinking and programming in 3D space. The second tutorial (which has many, many mental exercises that are actually ~fun~ 
is recommended as necessary by the first.
Math for Java Game Programmers, Getting Started (and the other tutorials in this series)
http://www.developer.com/java/other/article.php/3733176/Math-for-Java-Game-Programmers-Getting-Started.htm
Vector Math for 3D Computer Graphics
http://www.dickbaldwin.com/KjellTutorial/KjellVectorTutorialIndex.htm