Want to really dig deep into the inner workings of WordPress? Did you know that you can browse the source code online? Not only is it presented in a convenient web interface but it includes inline documentation so you can learn more about how WordPress works.
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Spend a few afternoons a week reading through the source and you’ll really get a good handle on WordPress classes and functions and the files where they can be found. If you ever have a question about how a particular bit of code in the core works, you can always refer to the online source that contains the inline documentation.