In the 1830s and 1840s two men on two different continents - Charles Fenerty (Canadian, 1821-1892) and Friedrich Gottlob Keller (German, 1816-1895) - began experiments with wood using the same technique used at the time in paper making but instead of pulping rags, they thought about pulping wood. At about the same time – around mid-1844 - they announced their findings. They had invented a machine which extracted the fibers from wood (exactly as with rags) and made paper from it. Fenerty also bleached the pulp so that the paper was white.
This started a new era for paper making. By the end of the 19th-century almost all printers in the western world were using wood in lieu of rags to make paper, causing a major transformation of the 19th century economy and society in industrialized countries. With the introduction of cheaper paper, schoolbooks, fiction, non-fiction and newspapers became gradually available by 1900.
Lignin is found in wood pulp and is the substance that exacerbates acidity in wood-based paper. Paper made from wood-based pulp that has not had its lignin removed turns yellow, becomes brittle and deteriorates over time. When newspaper turns brownish over time, this is due to the lignin causing acid to form within the paper fibers. When exposed to light and/or heat, the molecules in the acidic paper will break down even faster. This is what had happened before I had purchased it to my own copy of Henry Uhlig’s 75 Ideal Tropical Fish for the Community Aquarium, published in New Jersey in 1934. Mass-market paperback books still use these cheaper mechanical papers, but today much of the commercially produced paper is acid-free largely as the result of a shift from china clay to (cheaper) chalk as the main filler material in the pulp. Chalk reacts with acids and neutralizes any acids formed in the paper over time (The sizing additives mixed into the pulp and/or applied to the surface of the paper must also be acid-free.)
The best way to protect books is to store them in optimal environmental conditions: the relative humidity between 50-55%, a temperature between 68-70 °F and minimal light. Do not store books in attics, garages or basements, as these areas typically have large temperature fluctuations, high relative humidity and pest infestations. For books that have absorbed some moisture because of being stored under these conditions, stand the book on end, slightly fanned. If the cover is damper than the text, place absorbent paper after the covers and change them as needed. Use fans to circulate air to dry the books completely.
If your books are already deteriorating, there are a few ways to deal with acidic paper and some of the options cost more than others. The cheapest option is to make a preservation photocopy of the book and use it instead of the original. The original could be stored in a box for protection but it will continue to degrade. This, of course, will not satisfy hobbyists for which collecting old books is a hobby in itself.
Another option is to have the book treated by a trained conservator to de-acidify the pages. One method of de-acidification involves spraying each page with Bookkeeper, a product based upon a non-aqueous, magnesium oxide suspension system that will begin to neutralize acids in the paper. Sometimes, however, paper has become so brittle that virtually no treatment is possible.