Leonid Mamchenkov is writing a very useful blog called WordPress Bits: Hacking WordPress. Keeping the bits together.
His blog is for people who want to work with WordPress to make sites. As he says:
this web site is going to focus on in particular:
- WordPress customizations.
- WordPress development.
- WordPress internals.
- WordPress as web application framework.
- WordPress as content management system (CMS).
If you want to understand WordPress, how it’s put together, how to create a theme from scratch, how the database fits into things then I recommend WordPress Bits as a place to start.