How do you remove musty or moldy smells from books?

Although some say that you can't, and that once the smell is there, it stays forever, there are many things that you can do to improve the smell of books.

Definitely not all odors are permanent.

It helps if it is summer and of course how musty the smell is - but try placing the book with pages open in direct sunlight, but only for 1 or 2 minutes per open page, keep swapping pages. It takes a while, but often helps.

If you cut a used tumble dryer freshener sheet in half and insert one or two pieces in the pages of the book and leave aside for a week it should help

See the related link for some more ideas.