I'm digging through all my bookshelves.  I pulled a book out today and was trying to decide whether to post it or not when I noticed "splotches" on the back cover.  It's a cloth covered hardback.  I know it's never been wet -- it's just been sitting on the shelf for about 20 years! 

The cover of the book is black/dark grey and the spots are lighter colored and all over the outside.  It kinds of looks like splatter -- but not! It doesn't smell. 

I've since decided not to post the book and was going to keep it, but now I'm wondering if I should just throw it away?  If it isn't mold/mildew, what could it be?  And how in the world would it get there?  The books on either side of it look fine.  And the spots appear to be only on the back cover? 

This would be a total guess but, yes I would think time and high humidity could cause mildew...

Yes.  All it takes is a little moisture, especially for some of the powdery mildews, which is what your's sounds like.

I would toss the book and wipe down the shelves with bleach solution.

Better safe than sorry, I guess.  Darn it. 

Just another reason to hate living here.  :-D

Well, besides the humidity, fungal spores are also needed but, yes, it sounds like an ideal environment for molds and fungus to grow, says my botanist husband.  I'm sorry :(  Living near the river makes things tough.

In addition to wiping things down with Chlorox, consider a dehumidifier for that area, if it is feasible, which will help keep the moisture down too.  Increase the air flow too, with fans, etc. -- climate control is your best chance.

Good luck,

Catt