The number one rule is that you have to start paying attention to the design of everything. Don Norman's "The Design of Everyday Things" is a good start on developing this, and you really shouldn't miss it. But this applies to everything: when something's nice, figure out why. Notice typefaces, and form a mental library so you can identify them.
It's a bit of an odd path, but I'll also suggest, since you're interested in interactive systems, reading several great Human Interface Guidelines. Apple's OS X ones are my favorite (You might even want to get an older version, because they're a bit more general), but the Nokia Meego ones are worth a look too. Most others are too mediocre to teach you anything though.
Some other books I have to suggest:
Bringhurtst's "The Elements of Typographic Style." Definately read some kind of lighter treatment of type first, so you know humanists from geometrics, serifs from sans, high and low contrast, and so forth, but if you want to really get it, this is the book. If you go in knowing that some of what he's saying is opinion, this is the book you need to read.
The Universal Principles of Design: It's really not the best book, and there's some things, like the readability research, that's actually just complete crap, but it's an alright, quick overview of a few dozen concepts. If you understand the concepts in it, skip it, but as a first intro, you could do worse.
The Humane Interface: Other than Raskin's whining about the Canon Cat not being a hit for slightly too long, it's got some interesting concepts that all designers should understand.
Thoughtful Interaction Design: This one's very theoretical, and very heavy reading; it's really about a way to view the design process, and how it proceeds, more than it is about how to design. Don't read it until you've done a few projects.
The Visual Display of Quantitative Information: This is a great book on presenting information. Just ignore the ridiculous stuff about minimizing ink, and think of it as minimizing the cognitive load.
For a quick overview of a couple interesting concepts, you might try method.ac, reading the wikipedia pages on Fitt's Law and http://worrydream.com/#!/MagicInk and trolling through http://informationarchitects.net/blog/.
Stay away from ux.stackexchange.com, news.layervault.com, PSDtuts, et. c. It's almost all crap, about tools rather than actual designing. In general (very much in general), if a blog post on design is short, it's crap, 95% of the time.