Bookcases can be real assets. They can act both as diffussors and absorbers. However, the original location of this bookshelf was probably only acting as an absorber, and not a very effective one at that. By adding an additional bookshelf and placing them behind the listener we are able to get the full benefits of the bookshelf. This will also reduce the effects of the modes related to the length of the room, although they won't be very effective on the longer wavelengths.