Identify how extensive the mold growth is. If it has spread only to the cover and edges, you often can brush off the mold without difficulty. If it has spread into the contents of the book, the pages can be either treated with chemicals or treated with more everyday measures.
Find a place with a dry, cool environment where you can work. The ideal temperature is below 70 degrees, and the area should be well-ventilated, since you will be using chemicals. Allow the books to dry, if they are still moist. Once they're dry, remove any loose mold clumps with a soapy cloth or brush. Let the books dry again.
Dilute chlorinated bleach with water in a small dish. Apply the mixture with cloth to the mold spots. Rub a dry cloth on the spots to remove moisture. Be careful not to scrub hard. Instead, pat the pages gently.
For a less arduous strategy, sprinkle talcum-powder cornstarch in the pages of your book. Let the book dry under a fan for a few hours. You might also try fungicidal spray.
If you do not have bleach or fungicidal spray, place the book in direct sunlight. The sunlight eventually will kill the mold. Alternately, place the book in a plastic bag and then into your freezer. The extreme temperature will eradicate the mold. Remove the dead mold with a brush.
Apply either copper napthenate or chlorinated phenols to the pages of the books to prevent mold from forming again. Either of these chemicals will resist mold spores that might again latch onto the pages. Once this step has been completed, you have successfully preserved your book from the threat of mold, saving one of your treasured literary possessions!