I am a web developer who has been using the WordPress platform for almost a year now. I have a working knowledge of PHP & MySQL, along with a Decent amount of HTML 5 And CSS3, I can create and customize basic Responsive WordPress themes.
What I really want to learn now is more advanced theme development that I can start releasing themes on WordPress.org and the like. I also want to learn how to integrate custom theme options editable through the admin interface.
Any guidance to a book or other electronic resource would be greatly appreciated.