Chronic -- you're right, that's mold. Not too bad, thought. I had a couple much more moldy books (or was it mildew? -- I don't know the difference) that I treated by wiping down the affected areas with a sponge dampened with bleach and then following that with a "rinse" pass of the sponge damp with water. This removed/bleached much of the stain and seems to have neutralized the mold -- at least it hasn't returned or spread.
Why keep a moldy book? One was a copy of Poe that had belonged to my deceased mother, with a great portrait in it, and I wasn't about to throw it away.
I've heard that if you store a mildewed/moldy book next to one that isn't, that it will spread. I've always wondered if that were true, or if it was some condition paranoia started by ultra fussy collectors. My two mildewed books haven't (apparently) infected any of my other books --yet. I guess to be safe, I could isolate them in plastic bags.
Anyway, good catch on the mold -- I missed that in my poor condition fetishism.