Cognitive Psychology / UX (UI is not UX)
Understanding what and why you are designing something is essential. Design is problem solving, pixels and code are simply mediums in which you create the communication of the solution.
If you understand how the system works (people) you can easily traverse how to solve problems (getting people to do/act how you want).
There are extensive resources on Cognitive Psychology, Consumer Psychology, User Experience Design and Behavioral science. They are listed below.
HTML/CSS (Developing your UI and including interactions)
The reason knowing how to build prototypes is so important is to speed up and execute your exact vision into the hands of professional programmers.
Great designers are so detail oriented and particular about what they create that losing an element of the solution, in code, should be considered unacceptable. Telling someone how it moves is subpar to showing someone how it moves
There are numerous development resources listed below. Most are free / near-free. All are cheaper than a degree.
Learning to code (at least for prototypes) is going to do nothing but good for you and increase your value as a team member or freelancer. Learning Cognitive Psychology and UX principals is going to help you solve problems in a more efficient manner.
Here are a list of books / publications for development and UX:
How We Decide – Jonah Lehrer
Designing with the Mind in Mind – Jeff Johnson
100 Things Every Designer Needs to Know About People – Susan Weinschenk
Don’t Make Me Think – Steve Krug
UX Design – Russ Unger, Carolyn Chandler
The Power of Habit – Charles Duhigg