Use one of these methods to deactivate the mold. Place the book in a plastic bag and leave the mouth of the bag open. Put the book in the bag in the freezer for several days. You can also set the book in full sun for several days. Turn the book over and fan the pages from time to time.
Remove the dust jacket from the book. If it's a glossy jacket, use a clean, soft cloth barely dampened with isopropyl alcohol to gently wipe away as much as of the mold as possible. Leave it spread out to dry. If the jacket is matte, use a clean, soft cloth but omit the isopropyl alcohol.
Dust with a feather duster to very gently remove surface mold and dust on the book. Be careful not to grind the mold into the binding or the pages.
Use a leather conditioner specifically designed for books if the binding is leather. Apply some conditioner to a clean, soft cloth and wipe the surface of the leather clean. Use another soft cloth to gently buff up excess conditioner. Repeat if necessary.
Apply a few drops of isopropyl alcohol or lighter fluid to a clean cloth. Gently wipe the boards and spine of books that are not leather-bound.
Fan the pages of the book to remove surface mold. A light dusting of talcum powder may help dislodge the mold. Fan the pages until the powder dissipates. If the paper is badly damaged, take the book to a professional for restoration.