"Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime." - Chinese proverb.
In my book WordPress Responsive Theme Design Essentials I have covered all technical aspects of WordPress theme development. Whether you want to create the custom themes for your agency clients or you want to go pro and sell your themes on WordPress marketplaces this book will provide more than enough technical info for you to reach that goal!