It’s interesting to note that the book was written through collaboration between 15 people on the forum and then released to the public, for free. This is how they describe them selfs:
Founded in 2006, Natural User Interface Group (~ NUI Group) is an interactive media community researching and creating open source machine sensing techniques to benefit artistic, commercial and educational applications. It’s the largest online (~5000+) open source community related to the open study of Natural User Interfaces.
Visit the forum and download the book at www.nuigroup.com/forums