I came across this book on the end processing cart at S.U. (For those who don't know, the end processing cart is books for special collections which come to the lab for book plates or repair before becoming available to the public). Leather can be wet during the binding process. In fact, you want it to be so that it is flexible and will stretch tight on the book. After it has been bound and on the shelf for a while, this is what happens when water is introduced. (The white you see is a Japanese tear strip for a hinge repair).