The back panel looks suspiciously like the side panels. In fact, it's the same, except a little bit wider. Like the side panels, it's just a piece of plywood with the faux rails glued on. The shelves are even simpler; they're just rectangular chunks of plywood.
The two shelves and the back are all attached to the sides with screws that run through little wooden ears glued to the plywood panels. This makes them easy to remove and replace as needed during construction and especially later when it's time to stain and varnish the remaining parts.
