Recently came across this...take foam core bought at any big home improvement cavern and cut to the size of your bookcase...paint it and press the foam in place..to sort of spruce up and change out the color of your bookcase backing...easily changeable if you're so inclined...such a simply "why didn't I think of that" kind of idea.  I'm all kinds of into making a project NOT permanent but easily transitional into something else...
The lady  that emcees the Biggest Loser was interviewed and she had a similar idea.  She would cover one side of the foam core with a beautiful Christmas paper and switch it out for the Holidays.  Okay...now how much is foam core? Maybe scavenge some large pieces of cardboard from a furniture store maybe a refrigerator box?  This is on my bucket list!
smiles luv...
colleen