That is possible, but I would assume that a transformation would have to have taken place in the stone to keep it from crumbling. I would have to assume that a thin sheet of stone would be very brittle.
Perhaps thin sheets of
metal. We know that the denizens of the Land had some ability to work metal, so perhaps near-foil metals were used.
Unable to stop myself from a little research, I found
THIS site regarding writing before the invention of paper.
There are numerous possibilities, although perhaps the Old Lords
did use paper, just not paper born of the Land. I always pictured ancient tomes while reading of the wards.