Eek. Water damage sucks. I remember once after a huge rain storm I was working on my computer and heard a dripping sound. Water was dripping in through my closet. Fortunately it was a little water and was dripping on my LOTR minis. Plastic can handle water nicely.
I've got a suggestion in case the manga doesn't dry well. Hold on to the water logged books and then grab some scanlations off one of the anime bit torrent sites. I think that's justified as fair use.