Hi there,
here’s the plugin API: http://codex.wordpress.org/Plugin_API
and here’s the Function reference: http://codex.wordpress.org/Function_Reference
hope that helps!
I found a tutorial that showed me how to roll my own blog theme, it works, but I would like a better understanding of what I am doing.
Is there a book that explains it all?
The only book that deals specificaly with wordpress that I know of is Digging into WordPress, but there might be others?
Aside from the book, these tutorials are really complete, and explain in detail the process of creating a wordpress Theme:
http://www.wpdesigner.com/2007/02/19/so-you-want-to-create-wordpress-themes-huh/
http://themeshaper.com/wordpress-themes-templates-tutorial/
If you already know some php, undestanding wordpress theming shouldn’t be difficult. Other than that, the codex is always a good place to start.