I am planning on starting a website business and want to use WordPress as my CMS. I will be creating hundreds of websites and need the ability to make them look very different and non-standard issue WordPress. I have experience with running blogs with WordPress and am now interested in running full fledged websites that break away from the standard two and three column design.
From my understanding from browsing the Codex I will need to learn PHP, CSS, HTML (already know it), and become familiar with the WordPress engine itself. Am I missing anything? I want to be able to have complete control over theme design and content layout. I want to be able to know enough to modify and possibly create plugins from scratch.
My question is what level will I need to be at with CSS and PHP in order to achieve this goal? Will I need to know PHP programming inside and out? I'm not super familiar with CSS beyond simple webpage display manipulation like font colors, creating tables, and other simple stuff. I don't know how deep the language is as a whole.
I am willing to learn whatever it takes to be able to have great control over WordPress and its presentation. I just need to know how many books I should be buying on these subjects. Most cover beginning, intermediate, and advanced levels (at least for PHP). How deep down the rabbit hole should I expect to go? Any help would be most appreciated!