I had a brand new  (unread) book in my car and it has been raining for several days...it now has ruffled pages that looks like it was lighty misted with water!

It did not look like that before I put it in the car....

Sheila, this can happen...especially with the cheaper/recycled paper that is now being used for books.  They are thinner and therefore absorb the moisture easier than books made years ago. 

I don't find that surprising.  If your car is not airtight (and most aren't), you will get moisture inside your car.  We have had a very cold winter here in Oregon and one day when my windshield was iced, I went to get the bottle of water from my back seat to melt the ice and found the bottle of water to be frozen solid!

BTW, if you had put the book under a heavy weight right away, while it was still slightly damp, the pages probably would have gone back to being flat.  Keep that in mind if it ever happens again.