I want to develop WordPress themes and plugins. What do I need to know to do this?
I have knowledge of PHP, HTML, CSS and JavaScript. I have developed a few projects using these. What is the best place to start (except Codex), is there any book? Where can I know how the core of WordPress works?
Check out this neat infographic for a good introduction to the anatomy of a wordpress theme.
For Plugins:
There are two places where you can code for plugin in Wordpress.
1. functions.php
2. Create a folder in plugins folder, inside that folder. Create a blank .php file, and code there.
For Themes:
As you asked for learning wordpress themes.
Go to themes folder, try to get the structure and flow of wordpress default themes. You've to code within themes folder.