The best way of getting rid of mold is to put the book into a heavy plastic bag and seal it securely so no air or moisture can get in. Then put it in the freezer for a couple of days to a week or two, considering how far gone the infestation is.
When you take the book out, the ‘moldy stuff’ will still be on the book, but it will be dead. Now take a new paintbrush (a cheap one is fine) and just brush off the mold. Sometimes mold will have stained the binding, and the color won’t brush off, but it’s just color.
It takes a while, but it does work – kills the mold but doesn’t harm your book. I’ve done it a number of times.
I’ve been told that if your book still has a smell, put it in another (very dry) bag with a couple of handsful of cornstarch. Seal it up and put it somewhere cool – though not the refrigerator or freezer! – and leave it for as long as it takes for the mold smell to go away. You’ll have to open the bag every few days, remove the book and smell. When it’s clean, get out your paintbrush and brush the cornstarch away. Sometimes, if there’s been just a slight odor, you can get two or three uses from the same cornstarch. If it has an odor, toss it. You don’t want to infect the next book!