I got the idea to make this from other projects I've seen on here (Make a Book Box by Supernova's Child), where a book is cut up and made into a box. As an avid bibliophile, I just couldn't bring myself to cut up a book, so I found an old wooden wallet box and used that instead. Unfortunately, I didn't take any pictures during the process, so my instructions are text-only. If you have any questions, feel free to comment :)
My box came with a wood finish, so to make it look more book-like, I painted it white, and made sure the streaks left by the brush went in a direction to make them look like pages (aka parallel to the seam where the box opens).
Get out a sheet of craft foam (I used white, which works best under most fabrics, but if you're using dark fabric, you can use any color foam) and trace the top and bottom of the box on it. Cut them out. These will become the front and back "covers" later on.
Lay the 2 foam pieces down on a piece of fabric large enough to fit the foam pieces as well as the "spine" of the box, which should be placed in between the foam pieces (so you can see how much fabric the cover will need). Trim fabric as necessary, making sure to allow at least 1/2" of space around the whole thing.
Remove the wood box from the fabric, but leave the foam pieces in place. Fold down the rough edges around the foam, and hot glue in place. I folded down the long edges first, then glued down the short edges.
Hot-glue the "spine" of the box to the portion of fabric meant for the spine. Then, glue one side of the box to the foam. It should actually start looking like a book now. Glue the last cover down to the other side of the box. Voila!