I went to a resale store run by Habitat for Humanity. They had these old cabinet doors for $3! It's all about knowing where to go—the "thrift" store in a suburb will have prices much higher than those of a resale shop in a worse part of town.
I cleaned them and the floor where they're going to be set up with Clorox wipes. (Yes, this is a paid sponsorship. All college kids need Clorox wipes.) (Actually, not a sponsorship. But Clorox wipes are necessary.)
Then (and here's where it gets complicated) (just kidding) you stack the books and the shelves however you like. Just make sure it won't fall.
I haven't decked it out yet, but I love it already! (Here's a picture of it with things on it.)
I would do a step-by-step, but that's about all there was to it!