How can I remove mold from an old paperback?

Unfortunately you really can’t. When I worked in the library and we were weeding the collection, if a book didn’t pass the “sniff test”, we chucked it. If it was still a useful title in the collection, we would order a new copy.

@janbb has it correct.

If you had books that were in a flood (see article about flooding that wiped out the library at Univ of Hawaii), you can get huge industrial strength freezers and freeze-dry the books. But that’s a bit much for your pupose.

The other thing is that paperbacks are published on low grade absorbent paper. Real hardbound books are normally produced with much better, more dense paper fibers, meaning that moisture would be less likely to damage the book.

Bottom line: FUggedaboutit

Mix up ¼ cup bleach with 1 qt. water and pour into a spray bottle. Spray the book just enough to get it damp, then let dry.
While waiting for the next application, remove the bleach solution from the spray bottle and flush the sprayer with clean water.
If you can, place the book between some other flat heavy materials until the book has dried.

I once cut off the edges of a moldy paperback. That worked.

https://www.art-us.com/2020/03/18/how-to-remove-mold-from-books/