Sergio (@sergiosagu) posted a great document in the Orchestrator Plugin SDK-section of the VMware Communities:
“Should I create a new Plugin for vCO…”
…or use one of the generic Plugins for REST or SOAP to create my integration?
It’s absolutely worth reading, even when you don’t plan to integrate with vCO (but “only” develop workflows for your environment), because a lot of points can be transfered to other integration challenges (like integrating databases) as well.
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