Many serious learners have large libraries filled with books. especially in humid Japan, these books can quickly grow old and moldy. To prevent this, store books with a dehydrating agent, such as baking soda.
For books that already smell musty, sprinkle baking soda or corn starch over books, or put books in a sealed bag with a similar dehydrating agent. Remove the agent later, and the book will hopefully smell much better.
How does this work? The dry powders remove the moisture essential for fungi and bacteria to grow on books. This kills the fungi and gets rid of their odors.