please bear with me for the shameless personal plug - my Adobe Photoshop HTML Panels Development course (subtitled: Build and Market Adobe Creative Cloud Extensions) is finally available here.
It's a 285 pages PDF (and Mobi/ePub) about Panels coding and marketing – from the very beginning to advanced topics – plus a three hours HD video-series.
Since official documentation is scarce and/or sparse to say the least, I'm sure it's going to be helpful to anyone interested in that topic.
Please refer to the official website for extra information and sample content (I give away for free a 40 pages excerpt plus half of an hour of screencasts). I'm running a special offer through April 2016 for the launch, so if you dig Panels hurry up!
I'd also like to take the chance and thank all the forum people whose contributions over the years have been of great value - you've helped a lot!
Information on developing extension is rare indeed. This append may be considered Self-promotion which is not strictly allowed here. I believe there is a need for it to be here. It surely will answer question that have been asked here and some that will be asked here. Good luck with your book. You would have a bigger abundance if you write a novel next time.
Nice, but something wrong with the link.