Many books produced in the late 19th and early to mid-20th century were made using inexpensive wood pulp paper. High cotton content papers can last indefinitely, but wood pulp paper is another story. Paper made from untreated wood pulp releases formic, lactic, acetic and oxalic acids over time. Acid accumulation yellows and deteriorates the paper, eventually destroying it.
Large book archives, like the U.S. Library of Congress, use deacidification treatments to extend the life of books made with acidic paper. Usually deacidification involves neutralizing the acid by adding an alkaline component, like chalk, to the paper.
For home applications, there are spray-on products that neutralize the acid in wood pulp paper. If you think a book may be valuable, though, it's probably a good idea to consult an antique book conservator before treating it yourself. Book preservation techniques are always changing, and it's best to check with the experts before making any modifications that could reduce the value of your investment.