After becoming wet, a book may develop waves across the surface of the paper as it dries. These waves will follow the grain of the paper. With better paper, the waves will be shallow and difficult to see, although apparent to the touch. Books subjected to moist or humid conditions, like fog, can develop a lesser degree of rippling.
The example 1943 AQHA Stud Book and Registry became wet, dried with the wave lines shown, and all pages stuck together. This book is scarce and in poor condition, yet remains a useful Reading Copy as the pages have become unstuck with no losses to text.