IBM have released a new RedBookfor ICN. Not only is this in PDF format, but you can now download it in EPUB format. It's just a pity that Amazon Kindle doesn't support this.
As you'd expect, the RedBook serves as a good introduction to ICN, but also has some great information on how to create plugins and extensions, and how to package Dijits (i.e. DoJo widgets) into iWidgets, for use in Business Space and Case Manager.