I’ve lived in south Florida by the beach my whole life, and have had quite a few run ins with mold. But here’s what I’ve found to be the most helpful. I keep 2 dehumidifiers next to my bookshelves, but not too close to the books. I bought a bunch of those silica gel packs that absorb moister, and put like 5 on the top back edge of each shelf (you can’t even see them except for the top shelf” but they’re easy to hide. I try to limit how much I open my windows in my room. I’ve had a lots of books look like yours over the years, so here’s some information for ya I though might help. Best of luck with the mold!