Here's the before photo:
I looked around a bit for shelves that would fit in the corner, but of course ended up at everyone's favorite source for cheap solutions: IKEA! They didn't have shelves that were the right size, but I did score these brackets for only $3 each!
Next stop was the hardware store, where I was able to have them cut a 10' pine board into two shelves that fit the exact dimensions of the corner. Technically they're only supposed to make one cut per board, but usually if you do the measuring yourself and mark it for them (and ask nicely and maybe bat your eyelashes a little) they'll do an extra cut or two. Buying one long board and having them cut it is always less expensive than buying multiple small pieces of wood. The board was only about $12, which was even less than the fake wood shelves at IKEA.
Once I brought everything home, I realized I forgot to buy sandpaper again, but I decided to just paint anyway (why am I so bad about sanding??). The cut ends of the wood were a little rough, but it wasn't too noticeable after a couple coats of paint. I let everything dry overnight and then it was time to put them up!
And presto! Quick and easy corner shelves for under $25! I think the most difficult part of the project was deciding which books and "flub dubs" (a term my husband has taken to using, after he learned Abraham Lincoln used it to describe his wife's knickknacks) should go on the shelves.