If I lacked a formal CS education and wanted to understand more, I would focus on algorithms. They are the foundation of everything at the software layer. The two basic questions you will learn to answer:

  Is a certain solution to a problem correct? 
  Can we make it faster or more efficient? 

Understanding algorithms doesn't really matter unless you need to scale or be very efficient, but when that is important, a good understanding of algorithms will be invaluable to you and your employers.

Also, this is the only major difference I've noticed between people with a good, theoretical CS degree and those without one. All the other skills are about the same.