You really want to become master of WordPress then you need to read some of the finest books to understands internal structure of WordPress. I would recommend even you don’t want to get into php coding or plugin development but at least that will help you learn fundamental of WordPress. If you decide to outsource some of the customization work to freelancers you will know how much time it would take to finish certain tasks, setting the price right for your work, otherwise you have to learn hard way on getting some of your work done at right price tag.
I previously mentioned a book on How to design WordPress Themes, here is another great book that i would recommend on WordPress Theme design :
