What is the quicket way to pick up Joomla? Books, training etc? We want to train 10 Mid-to-Senior level Java developers in Joomla.
I find that I pick up new a framework fastest by reading tutorials and watching video screencast.
If u are trying to save time, dont start with official joomla documentation. Go to http://www.scribd.com/search?cat=redesign&q=joomla and read some short basic tutorials for a basic understanding. After that, the best approach is to install Joomla on local machine and play around for few hours, installing/modifying modules, components, installing templates and etc. Practice and only practice makes learning efficient. After that, when you will want to go deeper into specific topics, start reading more advanced documentations, forums, try to understand Joomla from the code behind. You may also use video tutorials for a basic understanding. Any developers won't have problems learning Joomla,as it's has fast learning curve.
For backend development, I like taking the core components and examining them.