Wipe away the clumps of mold with a fine brush or a piece of cloth. Add soapy water to the cloth to boost its effectiveness. Usually, these clumps can be brushed off without much difficulty.
Dilute bleach water in a small dish, with three parts water and one part bleach. Apply the mixture with a piece of cloth if the mold does not brush off easily.
Apply talcum-powder cornstarch or fungicidal spray to the pages. Make sure that the chemicals will not damage the pages of the rare book.
Place the book in direct sunlight. The sunlight kills the mold, and brushing it off should be simpler. You can also place the book within a plastic bag and into your freezer. The extreme cold temperature should also kill the mold.
Place particles of chlorinated phenols or copper napthenate in the book to prevent further mold growth in the book. These particles effectively eliminate mold growth on the pages.