Prior to about 1920, book were printed on paper that was made from cotton or other fibers. Important documents used vellum or other animal skin based paper. The process to make paper from wood pulp then brought down the price of paper but unfortuanely used sulfuric acid in the bleaching process. (at the same time a machine was invented to strip pulp from hemp. This process would have been cheaper than the wood pulp, and not use acid or other harsh chemicals....but we know what happened to that idea.)

The upshot is that books produced with wood pulp are dissolving away. The Library of Congress is actively scanning the entire collection. I've heard that some books are so fragile they are destroyed in the scanning process.

It also why Universities make graduate students buy that expensive paper: it's acid free.