If you are trying to save time, do not start with the official joomla documentation. Go to http://www.scribd.com/search?cat=redesign&q=joomla and read some brief basic guides for a basic understanding. After that, the best approach is to install Joomla on the local computer and play for several hours, install / modify modules, components, install templates, etc. Practice and only practice make learning effective. After that, when you want to delve into specific topics, start reading more advanced documents, forums, try to understand Joomla from the code behind. You can also use video tutorials for a basic understanding. All developers will not have problems learning Joomla, as it has a fast learning curve.