It is not something you want to see in or near your home. When there is flooding, humidity, and/or improper storage, mold can grow on everything we own.
Health concerns aside, mold can cause books to fall apart and give them a musty smell. If you don't remove mold from a book, it will spread to other books in your collection. There are ways to spot, remove, and prevent mold on your books.
There are a number of signs of mold growth on books.
There is a thin layer of white, black, or gray covering the pages of a book, or its surface, when mildew grows on a book.
There is nothing you can do at home to make sure mold doesn't come back. If you put the book in a sealed freezer bag first, you can rub the spots with alcohol.
The best way to prevent mold growth on books is to keep them away from the humid environment. Storage against an outside wall of the house is what this means. The humidity in the room where they are stored needs to be controlled. Air circulation can be promoted by running a dehumidifier and a fan.