Geertjan is on a roll: he just posted the first chapter of the NetBeans module development book on his blog. He always surpises me by his posts and always makes me say " I wish I had this when I was doing my stuff so that I wouldn't have spent as much time figuring things out on my own".
The first book on NetBeans that I've read is about NetBeans 3.5. Although that was really nice in getting me up to speed on NetBeans at the time, a lot of the things in it (especially on the module development side) don't really apply. It is a really nice book to help you understand the inner workings of NetBeans; however, at the same time, a lot of the inner workings have changed and it becomes pretty difficult to figure out on your own which one of the things in that book are still valid, and which ones aren't. From that perspective, the book that Geertjan has started would be invaluable in that respect.
Since this (hopefully) last semester I would try to focus a lot on my thesis project (which has to do with J2ME mobile video), I probably wouldn't have much time to spend on digging around the NetBeans APIs and trying to figure them out on my own. At the same time, if Geertjan continues to write this book, I could probably try to write a "parallel" set of examples that do the same things that he illustrates for Wicket and Click for Tapestry.