Hi. I am a designer that is trying without too much zeal to learn to program. Here go some advice, no idea whether it helps.
First, i believe that "looking great" and "being good user experience" are actually very, very different things. Designers are good just at the first. The second remains a dark art for programmer and designer alike.
For example, thinking about which user input you need or could do without, which order to ask for them and so on, falls i believe neither there nor here. Which effect you should apply to the input box is utterly immaterial, and i would venture that a competent designer can make any kind of use-process beautiful to look at.
As for making beautiful, i have found no way to develop this except by endless boring practice. The best illustrator is generally the one who has done more illustrations. Rules or techniques (as in crutches) tend to give good results in the very short term and become stifling very quickly.
If you lack the stamina or interest to simply draw, i guess this is not a bad thing. The other way to see the thing is that you'll eventually get better at it.
That said, designers do tend to develop a variety of manias and sensibilities that, well, seem to be related, vaguely, to their craft. If you're willing to try to get that, i would recommend a very, very old book. I know this goes directly against what you were looking for, but, again, that is my opinion: Symbols and Signs, by Adrian Frutiger. (Amazon has it, not sure if it is bad form to put an amazon link here... Also, i do only know the Brazilian edition so i can give no advice about that...). Maybe this book can come out sounding like full of rules, but that is not the point.
Another, more recent book that is also incredible, but even more paranoid (the guy goes pages and pages discussing commas, then semi commas, then colons, and i just love it!) is The Elements of Typographic Style by Robert Bringhurst.
Finally, designers always do end up getting a bit of cognitive psychology, and it is a good thing, but no self-respecting psychologist could purport to understand the human cognitive system, so i would say avoid the ones who are too sure of themselves like how-to guides to Gestalt, those are just gimmics. Find something that is serious about cognitive science, but not serious enough that you'll sleep, and that is it. You can always go back for more...