I find a lot of books in the rain & they sell at exactly the same rate as non rain damaged books. On some occasions even faster, as the price tends to be slightly cheaper. Rain damaged books are still good!
It’s all part of the object’s history and I would say that it makes the object more valuable as it carries that story. You’re part of an ongoing story which begins before you and ends after you, why do people covet “new” things so much that they are willing to destroy the real wealth of their ancestors in favour of enriching some business magnate who dangles a shiny trinket in front of them. “Out with the old and in with the new!” right, just dump the whole of history and start again. I’m sure it’s not important.
We are all curators of the objects created before us. It’s not something that happens elsewhere in dusty rooms with museum clerks, it’s you – right here, right now. All the objects around you are part of the story & the museum is your environment.
Anyway. . processing rain books can be a bit of a pain but I have a book drying ritual that works pretty well – stand them up in the airing cupboard / server room, then once they are bone dry – put something heavy on them to flatten out the pages a bit.
NEVER try to open or read a wet book, you will destroy it. Dry it first! You can dry soaked books that you fish out of the river, it doesn’t matter how wet they are it matters how you handle them whilst they are wet.