Moldy books! Yikes!
Storing books in a cool, dry, place is really important.
Most of the time the worst that will happen is that the books
will get a little musty, develop that "old book smell".
But there is also the chance that a book could grow mold.
This book was a victim of improper storage, but given that it was published in 1705, it is not surprising that it had a few years living in Virginia without the benefit of air-conditioning.
Old leather-bound can be particularly susceptible to mold growth, and while it is possible to clean them, it is delicate work. Leather can get very friable as it ages, drying out
and becoming powdery, and mold will just make it worse.
Mold digests the material that it is sitting on, "growing" into the substrate just as much as it spreads out as colorful fuzz on a book cover.
And since mold can be an allergen whether live or dead, it is always important to have respiratory protection when working on moldy items. In the lab I have a fume trunk that I can pose right over my work area, the exhaust fan draws all the air and mold away from the book as I work. It even has a light to help me see what I am doing!
The dry cleaning sponge lifts up the dirt and mold from
the surface of the book, and as the sponge gets dirty, I cut
those bits off so I am always working with a clean surface. Over the cover, and then through the text one page at a time, very, very, carefully. And when I am done, there is a clean, use-able book, ready for the reading room!