Before you treat mold outbreaks, try to determine what has caused it in the first place. You need to pinpoint whatever caused the mold to develop and then fix it, because until you solve that problem, mold will continue to grow no matter how often you treat the affected material. Look for a source of water, such as an unnoticed leak, broken window, or moisture along outside walls. They are a prime area for fungus growth and spore distribution. Clean with a normal fungus-killing household cleaner.
Active mold looks fuzzy and has a musty ordor.
Do not attempt to remove active mold, it must be dormant for removal. Attempting to clean active mold will result in stains that are often impossible to remove. Increasing the air circulation will dry most books and active mold efficiently. It would be ideal to have several fans circulating the area to accelerate the evaporation and drying process. Do not shelve or store books directly against an outside wall.