here's my recommendation for a quick start guide:
1. Joomla Programming by Joomla Press, written by Mark Dexter and Louis Landry, a must have for starter (
joomlaprogrammingbook.com)
2. Lynda.com Joomla 1.7: Programming and Packaging Extensions by Joe LeBlanc (
http://www.lynda.com/Joomla-1-6-tutorials/Joomla-1-7-Programming-and-Packaging-Extensions/73654-2.html), yeah its still 1.7 but still valid to 2.5
3. Andrew Eddie's the Art of Joomla Course (
http://learn.theartofjoomla.com) , its very detail but (unfortunately) not complete, you only learn about backend's MVC, no frontend component, module and,plugins. I think he's too busy to extend joomla and don't have extra time to build solid and complete course. But if you have extra cash, i can recommend you to join in, the snippet for Eclipse/Zend Studio user is really helpful to develop extension faster (i think the snippet is one of the jewel in this course), instead its one that i use daily
These are resources that i have/joined and help me understand Joomla Programming faster. Its gives you the general understanding. But when you want to build killer component, there still a lot of nitty gritty of Joomla Development that you can learn from this group, joomla videos over the net (see joomla day video or joomla world conference video)
Hope this help.