Getting to know Magento inside and out is a lifetime of work. Try getting started with these:
Official user guides
The official Magento user guide page at http://www.magentocommerce.com/resources/magento-user-guide is a wealth of useful information not only about the nuts and bolts of using Magento, but also the bigger picture for managing your online commerce strategies.
Recommended commercial guides
Packt Magento Beginners Guide
The basics of driving your new Magento site, learning everything from product attributes and the order life-cycle to managing customer relationships
Packt Magento Sales Tactics Cookbook
Before Ebay bought Magento, the official documenation was sparse and generally unhelpful, so communities grew to support development and administration. These are some recommended sites:
A Croatian company with a wealth of useful tips and advice.