Idea#1 will probably be more work than you think. You'd have to make a six-sided box out of your flexible material, and a 6' column of books would most likely crush the ones on the bottom.
Idea#2 is more workable.
a) you could get up to 12" thick white foam, or glue-up 6" thick stock and cut it to a 12"x12" x6' column, then carve, using either a hot wire or
electric knife the books,
OR
b) buy blue foam in 1", 1.5", and 2" thicknesses and cut random sized books, paint sides first, then glue together. Either method will not be vertically stable without a base of something heavy on the bottom (I'm thinking the bottom books could be stageweights?)
In either case, be sure to observe all
safety precautions when working with styrofoam: proper ventilation, respirators, dust collection, it makes a huge mess.
Might be easier in the long run to just obtain real, unwanted books. I've heard that Salvation Army and Goodwill for instance will not accept enclyclopedias, which seem appropriate for this play. Perhaps a "wanted" ad on freecycle.com or craigslist.com would get you what you need, and you can take them to a recycling center after the run. Or check with your local libraries about books they can't sell and intend to recycle.
Hope this helps. If not, I'm just a lighting guy who hasn't done
props in 25 years!