Draft of the new NetBeans platform book is available on safari.oreilly.com
. You can purchase it for online reading or buy a print version. It is a "rough cut" which means that the language is not polished yet, there may be typos, etc., but the whole content is there. So if you are developing on top of NetBeans platform this is the best resource I know of at the moment, full of information - explaining both the concepts behind NetBeans platform and important APIs.