Some functional areas of WordPress useful for future and existing projects.
Custom Post Types in WordPress 3.0
Extending Custom Post Types in WordPress 3.0
Custom post types in WordPress
Function_Reference/register_post_type
Custom taxonomies in WordPress 2.8
Introducing WordPress 3 Custom Taxonomies
Function_Reference/register_taxonomy
http://phpdoc.wordpress.org/trunk/WordPress/HTTP/WP_Http.html
Some common WordPress tasks and the most helpful resources.
Unix - in slightly more detail
Unix Course... with some detailed sublessons
Common Unix/Linux shell commands
WordPress source code (on Github) or Alt. WP source on Github
WordPress Q&A on Stackexchange