You can buy unfinished crates at most craft stores. Between sales and the popular 40% off coupons, you can easily pay less than $10 each for them. I don't recall the exact amount I spent on ours, but I believe it was in the $11-13 each range, worth a few dollars more to me since they were already painted and have those cute little corner brackets.
My handy husband cut the two crates in half with a guide on his table saw, making a total of four shelves.
To hang them on the wall he attached four brackets to the inside of each shelf using screws. He also used screws to attach the crate shelves to the wall, securing them with anchors.
I love the look, the functionality, and the simplicity of this project. The kids are pretty excited to have their personal bedroom libraries!