Last night (literally) I had the honor to present an introduction to vCO as part of the #vBrownbag “Automate all the Things” community podcast series. See the recording below, more information and future agenda on http://professionalvmware.com/vbrownbag-automate-all-the-things-training-schedule/
Earlier this year WaveMaker, our preferred tool to built custom web frontends for vCenter Orchestrator, moved from VMware to Cloudjee Inc, a spin-off of Pramati Technologies (http://dev.wavemaker.com/blog/2013/05/30/cloudjee-inc/). In September the new owner hold a nice webex about their future plans with WaveMaker. See for yourself:
These are really exciting plans, looking forward to give the new mobile features a try!
And more good news: Ed Callahan has published a book about “Easy Web-Development with WaveMaker”, which I absolutely recommend you to have a look at! http://www.packtpub.com/easy-web-development-with-wavemaker/book 
Brian Watrous, one of VMware’s most senior technical trainers (you know him from the videos on www.vmwarelearning.com/orchestrator, don’t you…), has published a new video about how to customize workflow icons in vCenter Orchestrator. For your convenience, I embed the youtube video below. Make sure you subscribe to Brian’s blog on www.vvork.info and his youtube channel, there is more to come…
Great work, Brian! Keep them coming! 🙂
In the current issue of the German “ix” magazin there is the first (of three) parts of a nice vCenter Orchestrator tutorial. Buy the issue in paper at your favorite newspaper store, or online (you can also just get the single article): http://www.heise.de/ix/inhalt/2013/9/112/
The authors of the article, Jens and Guido Söldner, are also teaching a two day instructor-led hands-on workshop focusing on vCO and vSphere automation These are taking place in Düsseldorf and Munich starting at Sept. 10th and Oct. 10th 2013.
Get details and book your place at http://www.amiando.com/vmworkshop2013.html
VMworld 2013 is starting tomorrow in San Francisco, as usual providing a real bunch of Hands-on Labs you can get your fingers on all the products.
And of course there is a lot of vCenter Orchestrator stuff represented in these labs (hey, it’s becoming a more and more important component within the VMware vCloud Suite!):
And you can find vCO in quite a few breakout sessions this year:
So if you are attending VMworld this year, make sure to check out these sessions (live, or afterwards on www.vmworld.com)!
If you are not attending: Register yourself at http://hol.vmware.com and check the lab HOL-OPS 07 to get a guided introduction in vCO!
Steve Jin: VMware VI and vSphere SDK
Steve Jin ( www.doublecloud.org ) focus is on the VI Java Toolkit. However the books also includes a great general introduction of the vSphere API and lots of UML-diagrams of the API data model. Both is very useful when you develop vCO scripts interacting with vCenters
see also
http://www.amazon.com/VMware-VI-vSphere-SDK-Infrastructure/dp/0137153635
David Flanagan: JavaScript – The Definitive Guide
Since all the scripting elements are written in JavaScript, you need at least one book with a JavaScript-reference on your desk.
(Of course, you can find examples all over the web, but in most cases searching in the internet costs more time and distracts more than simply open the reference book on the marked site 
).
see also
http://www.amazon.com/JavaScript-Definitive-Guide-David-Flanagan/dp/0596101996
Marijn Haverbeke: Eloquent JavaScript
If you are new to JavaScript, most of the books about it out there are do not fit for an vCO-guy because they contain too much web-related stuff.
An exceptional introduction to JavaScript NOT focussing on the web-stuff is "Eloquent JavaScript". The book is really good to read, and best of all: There is an online version available on http://eloquentjavascript.net which contains an interactive JavaScript console. So you can directly test the code examples online in your browser while you read:
The fastest way to learn JavaScript from scratch!
see also
http://www.amazon.com/Eloquent-JavaScript-Modern-Introduction-Programming/dp/1593272820
Cody Bunch: Automating vSphere: With VMware vCenter Orchestrator
http://www.amazon.com/Automating-vSphere-VMware-vCenter-Orchestrator/dp/0321799917