The best place to follow anything on Augmented Reality is http://www.augmentedadvertising.com/. This site has lots of information and links to the most relevant articles, books, and applications.
As Titanium becomes more commonplace, more training materials, reference guides, and community forums are appearing. But the only real place to get the latest information at the time of writing this book is www.appcelerator.com.
For more Open Source information, follow Packt at http://twitter.com/#!/packtopensource