Magento documentation is really building up. Here are a handful of key links.
Wiki: http://www.magentocommerce.com/wiki/
Knowledge Base: http://www.magentocommerce.com/knowledge-base
Design Guide: http://www.magentocommerce.com/design_guide
Magento Architecture: http://www.magentocommerce.com/wiki/doc/magento-architecture
Create new page/custom module: http://www.magentocommerce.com/boards/viewthread/2191
Customization: http://www.magentocommerce.com/wiki/how-to/customize_part_of_configuration
