The back is a thin cardboard that is thinner than the box the shelf came in.  It is made of veneered particle board and the front edge veneer is not smooth.  You can see the bumps from the particle board under it.  The upside of the material is that it is super light.  It does come with an L bracket to mount it to the wall, which is nice.  Time will tell how it is going to hold up to the weight of the books, but it is not sagging yet.  If you need something and can't afford more, this will work, but definitely not a lifetime piece of furniture.