I ask this because learning how to code on your own seems -- in a way -- much easier. Just go on the internet, look up the necessary resources et voila, done. But that's not the case with design. So how could one self learn design?
Learning design seems easy because the barriers to entry are almost unexistent: no matter how bad it is, most designs will somehow do the job. By the other hand, to get design skills that can land you a job might not be an easy task.
The scale changes the best way to approach design. It matters if it's small scale (web/graphic/product), medium scale (architecture) or large scale (urban) but - with different weights - usability, proportions, trends, psychology/sociology and economics are always relevant. Anyway a good designer will adapt to a different scale within a short time: design is design.
With design, maybe more so than with code, it's a lot about developing good taste. Code can be empirically worse or better but design is a lot of having a point of view and working within self imposed constraints.
One book I'd really recommend is "The inmates are running the asylum" by Alan Cooper. It's more about shifting how to think about design rather specifics, but it's one of those books that puts a voice in your head that will help guide a lot of decisions.